Diamonds are a Magical Girl's best friend. They are also friends to Mons, Doomsday Devices, space rangers, magical knights, Humongous Mecha, and regular old Robots. So if you want something to look highly advanced or incredibly magically powerful, just slap a Mineral MacGuffin on it; just like a normal MacGuffin, it doesn't even have to do anything to be neat!
You see while humanity has been fine for gold and silver to coast by on their prettiness to be worth something, in our fiction we seem intent on making jewellery work for their value and turn them into a universal battery so popular for energy storage it makes the AA look shameful.
In a magical setting, they'll often be called "The Eye of..." and Power Glows will be in full effect. In science fiction settings, they tend to be welcome with the Sufficiently Advanced Alien races. They also can appear in less advanced settings when doing the job of "focusing crystal", not directly making the energy themselves but somehow being able to turn any other energy source into Stuff Blowing Up by having lights bounce around off the internal angles. Your Mad Scientist or Evil Sorcerer will probably be using such a crystal to power his Death Ray.
In a Video Game, these gems are usually the Power Sources and weak points of the robots containing them. They also dim or crack when under a heavy battering, or sometimes even blink like a half-broken LCD display. Robots who are killed will frequently have their crystals dim and go opaque.
Power Crystals may be faceted or completely round and smooth. Depends on the design aesthetic.
Choice spots to bling out your power armor are:
Sailor Moon and Sailor Chibi-Moon's Power source and transformation devices are the aptly named Silver Crystal. They carry their respective versions in a brooch worn on the bow on their chests. The same crystal is also carried on Sailor Moon's wand during the first arc/season after it's obained and the other girls get a crystal they transform with in the manga. Each senshi is said to have one known as a Sailor Crystal (apparently the the Sailor Starlights carry their Sailor Crystals in their brooches acorrding to the Material's Collection).
In Vision of Escaflowne, the Escaflowne has crystals that seem to serve no function but decoration, and a chest-mounted crystal that's basically the engine — this is where the pilot places a dragon's heart in order to fuel it.
We actually see the shoulder crystals firing laser beams in one of Dornkirk's visions of a possible future where Van and Hitome 'rally the dragon' to defeat Dornkirk.
The Angels in Neon Genesis Evangelion have glowing crystal cores which cause the creature to shut down or explode as if their body was made of gelignite when damaged, depending on the needs of the plot. Evas more sensibly hide theirs under several tons of armour.
There are exceptions to this rule, however, as not only do some Angels, like Leliel, Bardiel and Iruel lack these cores, but in the original series, the Angels do not melt when the core is destroyed. There are also some, like Zeruel, who can conceal his core behind a protective membrane (or ribs in Rebuild) should his AT Field be breached.
GaoGaiGar and the other entries in the Brave series frequently have this.
The Tohsaka family specializes in a school of magic devoted to storing magical energy, typically in jewels. Rin does so by sprinkling her blood on her jewels every night, and uses them in battle by throwing them. The downside? Using the jewels generally destroys them, and those jewels are really expensive, so the Tohsaka family always has money problems.
Jetfire in Transformers Armada has "Lensman" hand crystals, but has never been seen using them.
Energon stars in Transformers: Energon (the sequel to the above series) were used by the Transformers to create energon weapons. However, the energon star would eventually run out of energy and vanish, along with the weapons generated by it. Energon stars (and weapons) were colour-coded by faction: red for Autobots and yellow for Decepticons.
Halfway through Slayers Next, Lina acquires a set of four Demon Blood Talismans from Xelloss, which she can use to amplify her magical power. She wears one on her collar, another on her belt, and the others on her wrists; they glow brightly when she channels their power. In addition, she also sports a pair of fake crystals in her shoulderpads, enchanted to explode when removed by a thief.
Skuld in Oh! My Goddess has two on the backs of her hands, and two on top of her feet.
Witchblade has a crystalline "eye" which is visible even in passive form (as a jewel on her "bracer") and glows red when aroused going to kick some ass. See?
Dragon Ball's Chi-chi has one of these in her helmet, used for lasering anything she doesn't like the look of.
In Naruto, Naruto wears a crystal pendant that belonged to Tsunade and, before her, to the First Hokage. It can be used to hold back the Nine-Tailed Fox possessing him, but when he reaches six tails, Naruto deliberately shatters the crystal.
SD Gundam has crystals being used in various forms. Musha Gundam makes much of a 'Gundam Crystal', an artifact which is often split into separate parts and handed down to successive generations of heroes. Some versions of the series also have the Mushas house crystals on their torsos, inside of which are their 'GunSouls'. Upon death these crystals exit the body to await reincarnation.
The various Alternate Universe Gundam continuities love this trope.
In Gundam Wing, all three Wings and the Epyon have large green "search eye" cameras on their chests.
In Gundam X, the green gem on the GX's chest was the receiver for the microwave power transmission system that powered up the satellite cannon.
Most of the GN-powered mobile suits in Gundam 00 have these, usually taking the form of one large crystal in the chest and at least one on each limb. They're supposed to be the storage units of the GN particles that make them go.
In Dragon Warrior (Based on the Dragon Quest games), the Red Stone and Blue Stone hold untold power that can only be unleashed by those chosen to guard them. Most importantly, the Red Stone has the power to set loose an all-powerful dragon, who can only be sealed up again by the Blue Stone. The Blue Stone is given to Abel, and while normally it's attached to his headgear, on one occasion he removes the stone and fixes it to a magic staff. Talia is given the Red Stone, which she wears on a necklace.
In Tenchi Muyo!, the gems that can interchangeably go either on the hilt of Tenchi's sword or on Ryoko's wrists.
In the three-episode Team Galactic arc of Pokémon Diamond and Pearl, Cyrus uses a crystal of the Red Chain in the palm of his glove to mind-control the legendary Pokémon Uxie, Mesprit and Azelf.
In Guyver there are the Zoa crystals which are embedded into each Zoalord's forehead that give them their power. Some even have other crystals stuck into other parts of their bodies.
Bellcross in Heroic Age has crystals on his chest, forehead, and palms... and making up his shoulder and hip joints.
Xam'd's title monster is formed by one of these embedding itself in a person's body. Bonus points in that the crystals are the souls of the dead (albeit after some sort of processing) and fully sentient.
The title character of Tekkaman Blade uses a green crystal to transform ("Tekset"). Other Tekkamen, mostly hostile, use crystals of various colours and shapes.
Brutally spoofed in Atomic Robo. While fighting Dr. Dinosaur, Robo articulates several reasons why a time-traveling dinosaur scientist is flatly impossible. After Robo notes that Dr. Dinosaur would have had nothing but fronds and rocks with which to build a time machine, Dinosaur angrily attempts to Hand Wave it by saying "I had CRYSTALS!"
The wizard Ogeode's Prisma-Crystal in Crystar Crystal Warrior. He needed it for pretty much any really powerful spell, and it was also the mechanism for turning normal humans into crystal warriors.
The Infinity Gauntlet from the six-part series of the same name required the empowerment of seven gems representing the powers of soul, time, space, mind, reality, power and ego.
One Mickey Mouse comic (a Scrooge McDuck/Mickey Mouse crossover to boot) dealt with the characters chasing after an alien babe who was stealing diamonds. As it turns out, she was needing them to fuel her spaceship. (There is a small Critical Research Failure moment when Mickey apparently figures out that the diamonds must be fuel, because "they contain carbon".)
It's been suggested several times in assorted media based on DC Comics characters that Kryptonite is a formidable power source. Generally done in the interest of making things harder for Superman.
Suggested? It was explicitly used as the power source for Metallo in his original incarnation, though with no anti-Superman malice at the time (all the scientist who made Metallo happened to have on hand when he made him was some Kryptonite)
Count Von Deadly in All Fall Down uses one of these— until he explodes.
Paul is basically one big human-shaped power crystal—his hair, teeth, and nails are all magical diamond, and his bones may be as well. And he got that way by unknowingly grabbing onto a Trap Gem, which turned him into a diamond statue.
John's Kansael is a blue teardrop-shaped diamond.
The skahs wizards cast some of their magic through diamonds.
The Tears Of Gaia: There is a very ancient magical artefact called the Tears of Gaia. According to the Creation Myth, it is a gem created by Gaia with her own shed tears and the powers of the dead Alicorns are stored inside the Tears. The power found inside can be enough to wipe out most of the world! Most nations and the Blight want it in their possession, the other nations for defence and the Blight to expand its influence faster.
Parodied in Napoleon Dynamite; the crotch-frying "time machine" that Uncle Rico ordered online needed you to insert the most vital part: power crystals. Which probably were just pieces of quartz.
In the Superman films, crystals activate the holographic systems in the Fortress of Solitude. They can also remove (and, in rare cases, restore) Kryptonian powers.
In The Hobbit, the Arkenstone of Erebor glows and pulsates with its own inner light, although the flashbacks don't show it powering anything in particular.
In The Dark Crystal, the titular crystal must be repaired by Gelfling hand to "set things right."
In Atlantis: The Lost Empire, the Atlanteans have small crystals with evident healing powers, and a big huge one that's under the city that keeps the ecosystem down there going.
Lightsabers in Star Wars utilize special crystals that are required to make the 'blade' of the saber. The crystals are found in caves on the planet Ilum and then imbued with power using the force though meditation.
According to Knights of the Old Republic, there are many planets where a potential Knight can get a crystal, and different types of crystal can affect the attributes of the lightsaber.
IIRC you can use any type of crystal/gemstone (and then some) for a lightsaber, just that the Adegan crystals are the most convenient for the Jedi. The gemstone Tenel Ka used for her saber is actually a silicon-based lifeform.
When the Jedi fell and the Empire rose, the Emperor had almost all of the known sources of lightsaber crystals destroyed and made crystals illegal to possess and trade. For his own purposes he used a special furnace to create the red lightsaber crystals he and his apprentices used. When Luke Skywalker had to create his replacement lightsaber he found such a furnace in Obi-Wan Kenobi's hut, along with instructions on how to build it.
The glowing sensor crystal at the base of the Ilia-probe's neck in Star Trek: The Motion Picture Spock, upon seeing the giant image of Ilia in V'Ger's memory, says "The sensor must contain some special meaning. I must attempt to mind meld with it." He does, finds out that it's a direct link to V'Ger, and the power of V'Ger's mind knocks him unconscious.
In Toy Story, Buzz asks Woody if they still use fossil fuels or if they have discovered crystallic fusion.
In Marion Zimmer Bradley's Darkover novels the "starstones" (aka "matrix cyrstals") can amplify a person's Psychic Powers to the extent where they can achieve magic-like effects. One character observes that a particularly large and powerful matrix could "pull one of the smaller moons down out of its orbit".
The time machine in The Time Machine is made mostly of quartz carved into a mildly impossible shape.
In the Inheritance Cycle, gems are capable of storing massive amounts of magic, allowing a mage or dragon rider to store days and weeks worth of magic, so that they have a massive supply at their side to use at once, as opposed to merely whatever is in their bodies. For this reason, dragon riders' swords are set with gems, and may carry extras around, as well. Eragon often stores magical energy in the jeweled Belt of Beloth the Wise, or else in Brom's ring.
According to the Young Jedi KnightsStar Wars novels, lightsabers are powered by a simple battery, converter, and "focus crystal." Apparently just clarity and sparkliness is required of said crystals, Jedi can "sense" if it's good or not. (They don't even apparently need to be of any particular mineral.) If they're not sparkly enough, the lightsaber will explode.
Other Expanded Universe sources expand on the idea. Lightsabers with multiple crystals can have more than one length setting. The crystal may be responsible for the color of the blade, though the colors of the crystal and blade need not match. Not all crystalline materials will work; in one case, Corran Horn constructed a blade with two length settings, but the extended length didn't work when he used a fake diamond.
In fact, it shorted out rather spectacularly, causing both Corran and his enemy to be extremely surprised...and causing his enemy to drop the deadman switch he was holding.
David Eddings appears to be fond of this trope... both of his main universes - the Belgariad and Sparhawk settings - features a gemstone with enough power to give GODS a case of the shivers. Fortunately, these artifacts - the Orb of Aldur and the Bhelliom Blue-Rose, respectively - are in the hands of the heroes. The Malloreon, however (a sequel to the Belgariad) also features an equally-powerful but EVIL gemstone, the Sardion.
The mages in Avalon: Web of Magic each have an item of jewelry that "channels" their magic. Also, the main plot of the last six books is finding the Nine Power Crystals of Avalon in order to stop the flood of bad magic.
Examples in the Shannara series include the Diapson Crystals which convert solar energy to power airships and the Elfstones, which have so many uses (sometimes unintentional) as to serve as a veritable MacGuffin for the series.
The Codex Alera series features a variant, in that the Canim bloodstones don't power their magic but protect the wearer from it. They are usually set in the hilts of their weapons or in amulets.
The currency in The Stormlight Archive is made of crystals that store Stormlight. On top of that, they are luminescent when filled with Stormlight, so many kings use them to illuminate their hallways. A very practical solution. Until an assassin well versed in magic comes for you. Anyone with Surgebinding abilities can breathe in the Stormlight held by the crystals, granting them enhanced speed, regeneration, and powering whatever their particular magical abilities are.
Shardplate is also powered by Stormlight, and it's common to encrust the suits with gems to enhance the armor's durability.
The magitek in Tough Magic is powered by mescs and macrys, both varitations on power crystals. Interestingly, they can, and in fact have to be regularly filled with magic, making them more directly similar to batteries than most examples.
Live Action TV
Ultraman uses this as an indicator as to how long his power supply has left.
Star Trek dilithium comes in crystals, and in the original series must remain in that form to work.
Later Techno Babble describes it as not the actual power source, but as a regulating agent for the Matter / Antimatter reaction (IE: what makes the energy usable for purposes other than blowing things up).
Land of the Lost had a lot of these, with varying effects from dimensional gateway controls to impromptu grenades.
YMMV of course on whether it's this or Real Life, but according to Edgar Cayce and others, Atlantis made heavy use of these, and it was part of what led to their downfall.
Many New Agers and neopagans believe crystals have special properties that enhance certain qualities or aid in healing.
Orgonite. It's not actually a true crystal, but molded resin (eg, plastic) filled with bits of minerals and shiny things. The purpose is to absorb bad "orgones," or negative energy. Whether or not they actually work, some peoples' orgonites are quite beautiful.
The Craftworld Eldar of Warhammer 40,000 universally wear "Waystones," a gem worn over the heart which captures the soul of the Eldar upon death to prevent it from being painfully devoured by the Chaos god Slaanesh. These spirit stones allow the Eldar a peculiar kind of necromancy with Wraithguard and Wraithlords, battle constructs controlled by the spirit of a long-dead Eldar in a waystone. The Eldar also pimp out their vehicles and weapons with countless more mundane gems.
They also occasionally have starships and titans powered by spirit stones. Though, since they already have a light cruiser class called the 'wraithship,' they had to call spirit-powered starships 'ghostships'
Their evil-er cousins the Dark Eldar have these too. They hold souls, but unlike the Craftworld Eldar, who use them to hold their own souls in stasis in death, the Dark Eldar use the crystals to hold the souls of tortured captives for various uses, primarily consumption.
Dungeons & Dragons are full of magic jewelry, both standalone and embedded in other items.
Dungeons & Dragons 3.5 introduced a class over the internet, whose schtick involves embedding crystals into their skin. Precious gemstones specifically.
Years before Dungeons & Dragons 3rd Edition came out, the Earthdawn roleplaying game featured not only crystal plate mail and shields, but also living crystal armor (small columns) and so-called "blood pebbles" (hundreds of small magical gems) that can be implanted into a character's skin and grow by feeding on the magic of the character's blood until they provide a natural armor bonus on the whole body (except the head). Because the crystal is alive and part of the character's body, the character can feel and move with it as if the crystal was his skin.
Many spells involved gems as component or focus (e.g. Gemjump - teleport to prepared gem). Sourcebooks on magic items and materials (like Volo's Guide to All Things Magical) have lots of gem-related entries.
Psionics has a particular affinity with crystal - most psionic items are made at least partially out of crystal, psions can acquire a "psicrystal" companion, and there are psionic powers that can summon shards of crystal to attack enemies with.
Not to mention "power-up" crystals embedded in armor and other items that make them quasi-magical.
Hearthstones in Exalted are magical gems produced via geomancy that provide their user with energy and special abilities when attached to an attuned artifact (weapon, armor, amulet etc.).
GURPS has "powerstones" (which are always made from gems) that are pretty much absolutely required to provide extra energy for casting any powerful spell as the only other alternative is horribly ironic death as the casting rips your life force out of your body.
The Indiana Jones Adventure has hinted within the queue that Indy became lost on his search for the tourists going after a Gem of Power that provided the energy source to the entire Temple of the Forbidden Eye
The Sorcerers of the Magic Kingdom game has Hades and his army of Disney villains seeking out pieces of the Crystal of the Magic Kingdom that could allow him control over the park and the Disney worlds within.
Zero's Power Crystal also has his "Z" insignia plus a little relevance. It might be his weakness, as a flashback in X4 shows that Sigma couldn't have defeated him if it weren't for Zero convulsing when it was flashing.
The red one in X's helmet has a couple of functions besides looking neat. If his health is low, it blinks on and off while he's standing still. In X3, the level maps you can access after acquiring the helmet upgrade appear to be projected from the gem like a hologram. In terms of gems that are just there for show, many of X's special armors have ones in various spots. In particular, his Blade Armor from X6 has green gems on his arms, shoulders, chest, waist, and the tops of his leg guards.
Samus has a "Lensman" hand crystal on her non-cannon arm. In Zero Mission, it corresponds to the Power Grip, and in Metroid Prime 3, the Grapple Beam is fired from it.
Some Pokémon have gems encrusted in them for show: Staryu and Starmie, Golduck, Persian, Espeon, Ampharos, Sableye, Vespiquen, etc. Suicune deserves special mention, as he has a big friggin' crystal encrusted in its head. There is also an attack called "Power gem".
Pokedex entries for Staryu/Starmie claim that their crystal IS actually their eye, which is based on "compound eyes" of some real starfish.
Appropriately enough, Dialga and Palkia have a diamond and pearl on their bodies, respectively. The events of The Rise Of Darkrai are set off when Palkia's pearl is damaged.
Emerald of the Manga has one on his forehead.
The crystal in Deoxys' body is actually its brain. In fact, as long as that is intact, it's still alive, considering its Healing Factor allows it to regenerate in the right conditions. Staryu and Starmie's center gems also share this same healing factor.
Each type has a gem that, when held by a Pokemon, powers up moves of that type.
The latest Nazi plot in Wolfenstein revolves around the so-called Nachtsonne Crystals found near Isenstadt, Germany. Interestingly, the crystals themselves hold no intrinsic power, but are instead used as "lenses" for the Black Sun energy leaking into our universe. The Nazis harvest the energy, storing it in special barrels, which they use to fuel various crystal-based contraptions. Different crystals have different results: some slow down time, while others are used to create Some Kind Of Forcefield or activate the Thule Portal.
The Savior from Devil May Cry 4 had blue gems on his forehead, his chest, and both of his arms. For best results in order to save Nero, Dante has to destroy them all, though only damage to the gem on the chest counts as damage to the boss himself.
The Jumi from Legend of Mana have gems as their hearts. It gives them unlimited life and lots of power, but if it's so much as scratched, never mind destroyed, bad things happen.
In Starcraft, the Protoss Khala relies on Khaydarin crystals, which can focus, store, and amplify their Psychic Powers and allows them to join minds. The pylons, the ones you need more of, are basically giant floating chunks of the stuff. The "minerals" that are harvested by all three races are also some kind of crystal.
Protoss units are also often adorned with power crystals. Starcraft 2 has some units, like the Warp Prism and Void Ray which are built around them. The fluff describes the Warp Prism as a "computer built into the lattices of a crystal".
This gets amusing in Starcraft 2, when a Protoss crystal is brought on board the Hyperion and studied in its lab. The crystal grows and eventually begins emitting a lot of power, but the lab tech cannot figure out where said power is going. Turns out the crystal had tied itself into the ship's power grid without anybody noticing, and is now helping to regulate the ship's systems and keep them all at optimal performance levels. Also, it's probably sentient.
The all-important Spirit Gems that Jeanne d'Arc's heroes wear on their armlets. Each Gem grants a new transformation (each character can wield between three and five, depending on the slots on their armlet) and two of them can be Lost Forever if the player doesn't have sufficient warning.
The Chaos Emeralds from the Sonic the Hedgehog games are the epitome of this trope. There are seven of them, and with even a single fake Chaos Emerald one can teleport. A single real one is all that it takes to essentially freeze time in a decently sized area. Two Chaos Emeralds are capable of opening a portal through time. And when you get all seven, its pretty much the epitome of Super Mode.
The Elemental Crystals from various Final Fantasy games. Their exact powers vary from game to game, but they're usually the main Cosmic Keystones of the world, maintaining and controlling the four elements. In some games, their powers can be harnessed and amplified, and in a few, even a single shard of a broken Crystal has enough power in it for a person to transform into a hero.
Specifically, in Final Fantasy XIII, crystals are the source of magic: large crystals power the fal'Cie machine-gods who in turn bless humans with the power of the l'Cie by marking them with a Power Tattoo containing a crystal which grants them potentially limitless power... as well as a time limit and a specific mission. Failure turns you into a shambling monstermade of crystal. Success isn't much better as you are frozen in crystal until the fal'Cie or the gods see fit to de-crystallize you and return you to their service.
In Final Fantasy Type-0 each of the four nations bears a crystal gifted by fal'cie gods, these crystals in turn imbue the peoples of each nation with a different power. For Rubrum the power of magic but only to the young, the power of weapons to the people of Milites, the power of dragons to the people of Concordia and the power of the shield to the people of Lorica. These crystals can also imbue people with the power of the l'cie giving them unique abilities and eternal youth at the cost of degrading their sanity and absolute loyalty to the will of the crystal.
Amarr ships in EVE Online primarily use lasers. Fitting different crystals to the laser turrets affects attributes such as optimal range, damage type, and even what color the laser is.
Naaru Magitech in World of Warcraft seems to rely on these, at least judging from the amount of large crystals in the Tempest Keep instances. The Naaru themselves seems like sentient PowerCrystals / Energy Beings. The draenei use a lot of their crystals too.
Un'goro Crater has PowerCrystals of various colours scattered around the zone, as well as pylons that give various buffs when you bring crystals of correct colour to them. Several other magical crystals appear in the game too. Un'goro's crystal pylons are part of a huge homage to Land of the Lost.
Kael'thas Sunstrider in Magister's Terrace has a huge demonic crystal stuck in his chest.
Let's not forget the Jewelcrafting profession, the whole point of which, besides creating wearable stat-enhancing rings, necklaces and trinkets, is the cutting of gems able to enhance the stats of weapons and gear you socket them into.
Magical artifacts like the three spiritual stones in The Legend of Zelda: Ocarina of Time are often ornate gemstones with gold decorations or the like, while in the same game the gilver and golden gauntlets have red gemstones set into them, possibly the source of the extra strength they give you. In addition, the Master Sword, in all its appearances, has a yellow gem set in the hilt where the blade meets the crossguard.
Roland from Luminous Arc 2 has a Lensman-style artificial crystal on the back of his hand. It turns witches into his magic-giving harem.
In Tales of Symphonia, Exspheres are crystals that can be used to power machinery, and also enhance people's fighting abilities if equipped properly. Just sticking it on the skin has a ratherdifferenteffect, though...
Many of the more recent Tales Series games include these, usually enhancing each characters abilities, though some cases are more relevant to the plot than others. They're usually assumed to be naturally occurring crystals that just sort of appear in the world. They never are, and are usually either Powered by a Forsaken Child or some form of Lost Technology.
Tales of Vesperia has Blastia, specifically the Blastia cores. While Blastia is mainly used to augment each character's abilities, they're also used to power ships, barriers, and even act as mundane objects like thermometers or light sources.
Tales of Xillia has Lilium Orbs, which are, again, used by the characters to enhance their abilities, though they carry very little plot significance. There are also spirit fossils, used to power weaponry and flying vessels.
Tales of Graces has Cryas, which are used to transport water, heat buildings, and so on. There are also three massive Cryas known as the Valkines Cryas, which are generally used to power each nation's massive capital city by itself.
Timonium deposits in Rise of Legends look suspiciously like massive crystal formations
Fun fact, these crystals are used for EVERY RACE, including the non-technology based Alin. It is stated on the site itself that Timonium has incredible properties, including mystical ones (so presumably, Alin use it to power spells. Doesn't explain why the Hero Unit just use Mana instead.)
In Rudra No Hihou, each of the heroes chosen by destiny gets a jade lodged into their forehead. The impact ends up killing one of them and sending him straight to the afterlife.
Halo has the needler a pink gun that shoots pink crystals that don't do much damage and is kind of slow. Wouldn't be your weapon of choice right? Well if the enemy has enough crystals in his body they explode, all of them, at the same time. Also if the victim is carrying plasma grenades expect a big explosion.
Avernum features these a lot, particularly in relation to a): magical portals, b): golems, and c): anything made by the Vahnatai. The bigger ones often go boom nicely.
From the same developer, Geneforge features crystalline "power coils" as power sources for heavy equipment. These always explode if you screw up when trying to shut them down. Pre-destroyed ones are often surrounded by "flawed crystals" that function as Vendor Trash.
Oblivion had crystals all over Ayelid Ruins, some of which can be taken and used to completely refill the Player Characters magicka or recharge all their magical equipment.
Used as a power source for the ATACs of Vanguard Bandits. They're small enough to fit in your hand, but large enough to power a machine larger than a house.
In BlazBlue, Ragna the Bloodedge possesses a pair of red crystals on the back of his hands. They act as the activation devices for his Azure Grimoire.
And Kusanagi has large plates all over her battle suit. For the exact same reason as Ragna. Save that hers are more effective since she is the Inheritor of the Azure.
In Dark Souls, the Primordial Crystal gave Seath the Scaleless true immortality. Also, the most powerful sorceries are crystalline, invented by Seath after centuries of studying said Crystal. Fortunately, just because the Primordial Crystal is powerful does not mean it's very tough: it can be broken by a single attack.
In Kingdoms Of Amalur, Prismere crystals are used to make all sorts of Magitek for the human races, and the Tuatha use it to make weapons and implant it into their skin. It's seemingly perfectly safe for mortals but drives Fae insane.
All of the technology in Atlantis The Lost Tales, mainly flying boats called flyers, is powered by crystals. Crystals also have other abilities in the sequels.
In Dragon Age: Origins, the golem character Shale uses crystals for both attack and defense rather then use regular weapons and armour.
Strong Bad of Homestar Runner has a "blue diamond kinda thing" on his forehead which gives him the power to pop caps off of things.
In the world of RWBY, there is a crystalline substance known as Dust which was pivotal in humanity driving out the Grimm and establishing society. However, the series takes it Up to Eleven; Dust not only powers people's weapons, but also acts as fuel for vehicles and ammunition for firearms. Hell, it can be weaved into clothes and even be directly fused with a person's body.
In Gunnerkrigg Court, many of the Robot prototypes have large blue crystals (or possibly just panels of glass) set in their chests. For whatever reason, the production run of the Robots follow a less ornate design and lack the crystals.
The cybaspheres from the web fiction serial Dimension Heroes prove to be invaluable trinkets that give the Dimensional Guardians the power they need to defeat the more powerful opponents they face.
There are power gems in the Whateley Universe: Nightgaunt has two on his belt that he uses to shadow-walk; Splendor has several that she bought using daddy's money so she could have superpowers. It is mentioned that Mad Scientist types and wizards can with great effort and expense create 'cultured' power gems, a less potent but still useful form of the other type.
The crystal from The Quest. It allegedly grants you every super power there is, and gives you control of an army of some kind.
From the end of Beast Wars up to the end of Beast Machines, Transformers toys had 'spark crystals', which were translucent plastic gems that had the faction symbol of the character in them. The Transmetal II Blackarachnia toy was special, since it had a spark crystal which could be flipped to display one of the two main faction symbols, reflecting the character switching sides in the cartoon.
You can do a lot of things with Allspark fragments in Transformers Animated. Overload factory equipment, turn regular vehicles into Transformers, bring Transformers back from the dead, power bombs, and putting your Forgotten Superweapon back online is a snap!
In many Transformers continuities, though probably most prominently (first?) in Beast Wars, raw Energon—fuel for Transformers—is depicted as taking a crystalline form.
Storm Hawks features a world where everything is powered by crystals, and there's a crystal for every effect imaginable: hypnosis, explosions, "nitro-boosting" carriers, powering energy swords and boomerangs.
The main plot point of "Dressed to Kill," an episode of My Life as a Teenage Robot, was that an extremely powerful crystalline substance was being used in Brit and Tiff's commissioned clothing designs, giving them abilities that they would abuse inevitably.
The Mega Man animated series had shrink rays powered by red jewels, though Dr. Light found a way to alter them to do the polar opposite.
Robert Mandell, creator of Princess Gwenevere, is the king of this Trope as far as American animation. In his other series, Adventures of the Galaxy Rangers, Starstones and Psychocrystals were dark twins of the Riders' jewels.
In The Smurfs episode "Handy's Window Vision", the Smurfs discover a magic crystal that can transmit images exposed to it unto a bunch of smaller crystals that act as receivers, thus becoming their latest invention, the window vision. Gargamel, however, sees the crystal and uses it as part of his Smurf Smasher machine.
The Elements of Harmony in My Little Pony: Friendship Is Magic take the form of golden necklaces (and one "big crown thingy"), each one encrusted with a large jewel related to the bearer's Cutie Mark (so a star, a diamond, a lightning bolt, a butterfly, an apple, and a balloon for Twilight Sparkle, Rarity, Rainbow Dash, Fluttershy, Applejack, and Pinkie Pie respectively).
The Alicorn Amulet, an Artifact of Doom that corrupts the wearer, is encrusted with a large, blood-red gem. Similarly, the necklace that Twilight later dons in her rematch against Trixie features what appears to be an emerald; when it is revealed that the necklace is fake, this trope is invoked when the jewel promptly falls off and breaks.
The Crystal Gems from Steven Universe get their powers from the gems embedded in their bodies. They use them to materialize their weapons, and also to activate their technology (like the crystal laser cannon seen in the second episode). Magic crystals are also seen on occasion, such as the crystals shards that bring garments to life in "Frybo".
In Futurama, Al Gore's head offers a bag of "moon sapphires" to the first scientist who can solve global warming once and for all. According to a dark wizard in the audience, they could be used to open the Gate of Kerash. What the Gate is and why a dark wizard would want it open are left to the audience' imagination, but it's presumably magical.
Semiconductors, mainly silicon, germanium and various metalloid or nonmetal compounds called III-Vs, are functional crystals whose electronic properties make them extremely useful in a whole lot of roles. This includes use in electronic logic components like diodes and transistors, optical devices like LEDs and lasers, and - in a case of literally being power crystals - solar cells. Every integrated circuit (including your computer microprocessor and memory chips) is essentially a semiconductor crystal, that's been grown, cut, processed in a kind of arcane technological rune magic, and programmed, giving it the ability to create... all this. Although they have the same microstructural shape as diamond, most of these crystals don't actually look that pretty. But they are powerful.
Quartz crystals, a form of silicon dioxide, essentially the stuff of sand. When under pressure it produces an electrical current and it can be set to vibrate to fairly precise frequencies which led to its widespread use in governing electronic clocks, and generating signals for synchronous electronics (basically all modern digital systems).
Tourmaline crystals, like quartz, have the property of "piezoelectricity": when compressed, they develop electrical charges at the ends.
This is used to make lighters, although friction-based lighters are more common.
And for all of the above, you can thank Pierre Curie, Marie Curie's husband, who did the initial research on this with his brother Jacques. As teenagers.
Sapphires and rubies (more rarely, diamonds) are commonly used as bearings in high-quality machinery, including mechanical watch parts. Because of their extreme hardness, they can grind for years and years and not wear down.
Solid state lasers. Confusingly, these aren't the same as semiconductor lasers, instead having a (somewhat) transparent ion-impurity-laced crystal as the lasing medium with an external light-based power source (another laser or a flashlamp) instead of direct electronic power (usually). Indeed, the first laser ever built was a ruby laser, and today sapphire and garnet lasers are indeed pretty common for various applications.
Some New Age and Feng Shui practitioners believe that certain types of crystals have special magical properties. This leads to "crystal healing" with placing various minerals on a person's body, and putting crystals in water which is then treated like holy water (eg, used for rituals). Some people even "charge" their crystals by exposing them to sunlight or moonlight.
It gets a bit extreme for some New Agers, who warn that even small quartz crystals emit such powerful energy that they will burn your hands. Somewhere, A Geologist Is Laughing.