In fiction, we all know that anything can be a weapon. What's also true is that a weapon can be made of anything, including glass, something known more for its fragility than anything else.
This isn't quite as improbable as it initially seems; before metalworking, glass was one of the most popular materials for making weapons with sharp edges, along with flint and obsidian. Even today, glass knives are used in incredibly precise operations, since glass edges don't degrade as quickly as metal does, and it can be sharpened so much that it can create real-life examples of Absurdly Sharp Blade.
However, that doesn't get around the unmistakable fact that glass is incredibly brittle. It's one thing to use it as the head of an arrow or spear, but anything like a sword or dagger that's meant to block as well as attack would leave the wielder with a shattered weapon before long.
Not so in fiction, though! In fiction, anything from solid-glass swords to outright hammers made of glass are possible, often with the handwave of magic, which is why it's not uncommon for them to have magical properties themselves. These items still tend to be frailer than the regular versions, but nowhere near as fragile as they would realistically be.
An advantage that is sometimes claimed for glass weapons is that they can be hidden by immersing them in water. This would not work as well in real life, as water and glass have an index of refraction of 1.33 and 1.5 respectively. Corn syrup would be a better refractive match, but also kind of messy.
Grievous Bottley Harm is an improvised version, where a glass bottle is used as a weapon. Contrast Soft Glass, where glass is much less injurious than it should be, and compare Crystal Weapon for another type of brittle material unrealistically used for weaponry. Also not to be confused with Glass Cannon, which is a term for anything with high offensive power but very little defense (although, it is named for what would be an example).
- In Sherlock Holmes (2009), Lord Blackwood attempts to stab Watson with a Blade Below the Shoulder made of glass in the opening scene of the movie. He's saved from an Eye Scream only because Holmes is, of course, paying attention to his surroundings.
- In Daredevil, during a fight between Daredevil and Bullseye in a Catholic church, Bullseye picks up the shards of a shattered stained-glass window and hurls them at Daredevil like throwing stars.
- In Moonraker, James Bond's battle with Chang leads to a museum of glass art. They end up trashing the place and using the various art pieces as weapons. Bond manages to chop Chang's wooden katana in half with a glass-handled rapier.
- In Snow Crash, the Big Bad Raven uses glass knives that are Sharpened to a Single Atom because they're undetectable by security systems and can cut straight through bullet-proof vests. He also uses glass-tipped harpoons.
- In the sci-fi short story by Comte de Villiers de l'Isle-Adam, "L'Etna chez soi", there are very detailed descriptions of glass arrows loaded with chemicals that then explode on impact.
- In Mistborn: The Original Trilogy, glass knives are commonly used against Mistings and Mistborns as they can detect and control metal items. The State Sec's dreaded Steel Inquisitors often use obsidian axes for the same reason.
- There's a John Dickson Carr story where a killer hides a glass knife by dropping it into a jug of water.
- In Larry Niven's short story "What Good Is a Glass Dagger?", it turns out that a glass dagger has one significant advantage: it can be hidden in water.
- Spider Robinson's Callahan's Crosstime Saloon short story "The Law of Conservation of Pain". One of the guest characters has a futuristic gun made of glass. Jake Stonebender shatters it by playing a high frequency note on his guitar.
- The Obsidian & Blood series by Aliette de Bodard, being an Urban Fantasy series set in the pre-Columbian Aztec Empire, frequently features obsidian weapons.
- The first-person narrator of Death's Head mentions small glass knives early in the second book. They can be bought by the dozen and the user can easily break the handle off after stabbing, making the blade hard to dislodge. The narrator contrasts them with his opponent's enormous knife, the mark of a guy who takes pleasure in killing painfully (whereas he, as a soldier, kills efficiently instead).
- In The Stepsister Scheme by Jim C. Hines, Danielle's glass sword contains her mother's soul.
- In one of the Bony detective novels by Arthur Upfield, the victims are killed with coloured glass daggers that were once used as props in a magic act to which characters are connected.
- In the Eighth Doctor Adventures novel Demontage an assassin is disguised as a wine-glass salesman, and his Scaramanga Special (comprising two knives, a single shot gun and a set of lockpicks) is therefore made of glass.
- Dungeons & Dragons:
- Glass weapons can be considerably more practical in D&D. The 8th level "Glassteel" spell can permanently make a glass weapon as tough and strong as steel.
- In Dark Sun, metals are scarce, so obsidian is a typical material for cutting and piercing weapons.
- In the 5th Edition module Lost Mine of Phandelver, the rogue wizard known as Glasstaff wields, naturally, a Magic Staff of hollow glass, enchanted to be as hard as oak and to carry spells of shielding.
- In Changeling: The Lost, fetches can learn to pull a glass blade out of any pane of glass they can touch, with mirrors giving the best result.
- In Magic: The Gathering, a literal Shard of Broken Glass is a piece of equipment creatures can equip themselves with.
- In Battle Realms, the swordsman unit can be equipped with a battle gear upgrade called "Glass Sword", which could instantly kill another unit, but also sacrifice the swordsman in the process.
- In The Elder Scrolls series, "Glass" is one of the series' Fantasy Metals and is an iridescent green in color. Similar to obsidian, it can be mined directly (mostly from around the Red Mountain volcano in Morrowind, though deposits can be found elsewhere). It can also be smelted artificially by by combining refined moonstone and refined malachite... though admittedly they aren't exactly glass, per se. While the game refers to them as glass, they're actually malachite (not to be confused real malachite). In every game since Morrowind, Glass weapons are a high quality weapon type available in everything from swords, to axes, to bows, and even maces (in addition to being crafted into high quality light armor).
- Fire Emblem: The games Mystery of the Emblem, its remake New Mystery of the Emblem, and Fire Emblem Awakening have a subset of weapons made of out glass. They break after three uses and they're as strong as the fairly powerful Silver weapons, but they also don't require a high weapon rank to use.
- Brandon, one of the psychopaths in Dead Rising 2 uses a large shard of broken glass as an Improvised Weapon.
- Dark Parables: The Final Cinderella has the Glass Wand, a Magic Wand that's been passed down through generations of Godmothers that can destroy evil and break curses on objects, as well as imbue an object with positive energy. As a very major enemy in the game is a Perverse Puppet, this is a very useful weapon to have.
- Crypt Of The Necrodancer has glass weapons that do very high damage but break if you get hit. There's also glass armour that negates damage but (again) breaks if you get hit and an item called the Glass Jaw that lets you inflict double damage but turns you into a One-Hit-Point Wonder.
- Rise of Legends has the Glass Cannon unit, which is a cannon made of glass. As it lacks much offensive power (and is, if anything, rather sturdy for an artillery piece), it doesn't, qualify to be a Glass Cannon as in the trope.
- In The Binding of Isaac: Afterbirth, you can create the Glass Cannon, which is exactly what it sounds like, in every sense. It's incredibly powerful, but lowers your health to just 1/2 heart, meaning that you'll be killed in a single blow if something hits you.
- The Ultima series had glass swords from Ultima V onward: one-hit-one-kill weapons for practically every enemy in the games that shatter beyond repair upon a single use.
- Makai Toshi SaGa and SaGa 2 include the glass sword, an incredibly powerful weapon which breaks after a single use.
- In RuneScape, an obsidian sword, knife, and mace are found and used to unlock a door in one of the quests. They aren't, however, used to fight.
- Tibia has an obsidian lance as a dropped weapon, and in a quest it can be combined with steel to create an obsidian knife.
- A Very Long Rope to the Top of the Sky: There's the Glass Sword, whose Flavor Text is: "While not physically strong, the glass allows it to more easily channel magic."
- Warframe has Gara, whose powers all revolve around glass, either using it to kill or protect. Her update also introduced the Volnus (a hammer made of a glass-like material), the Fusilai (throwing knives made of glass) and the Astilla (a shotgun that fires glass slugs).
- Fable I has obsidian weapons, which are a notch up from steel in the Elemental Crafting ranks but which act like metal in every respect. Flavour text also says that they have an unsettling appearance, a pervasive chill, and a reputation for being used in evil deeds, although this has no effect on their in-game use.
- The Aztec maquahuitl was a length of wood with shards of obsidian, volcanic glass, embedded into the edges. Reports claim that it could decapitate a horse. They also frequently used obsidian to make knives and arrowheads, including sacrificial daggers.
- In a Real Life inversion, some scalpels are made from obsidian, especially for extremely delicate surgeries on parts like the eyes and brain. They're said to be sharp enough that they can be used without anesthetic. They're not all that commonly used, however, due to their unfortunate tendency to leave behind tiny, wickedly-sharp fragments. Ouch.