- The Lunarians for starters - inexplicable, otherworldly beings who seem to literally tear their way in through the sky, and never lose their pleasant expressions whether they're shattering and kidnapping gems or having their heads sliced off. Unsurprisingly, they've been compared frequently to Evangelion's Angels or The Watchers. They're eventually revealed to be more or less ordinary ghosts, even living a semblance of everyday human life when they return to the moon after a battle, but at the same time it almost serves to underscore the extreme cruelty of their actions. The Prince in particular doesn't bat an eyelash when describing how they've been grinding captured gems into dust.
- There's no gore when a gem is shattered, but most of the time this just makes things more disturbing; the first time Phos is cracked from the force of Adamant's shout, for instance, we're treated to the shot of them on the ground, staring straight up, eyes wide open in a cracked face. Eyes particularly feature in a lot of the shatter-induced Nightmare Fuel, actually; not only does one of Antarc's intact eyes fly out (in slow-motion in the anime) when they're shattered, but later the Prince digs out one of Phos's eyes with an arrow to replace it with a synthetic.
- Ventricosus' first appearance. Standing in front of a giant sea-snail is one thing, being digested alive by a giant sea-snail while fully aware of it is a whole other level of terror. Especially when the gems discover how they can't hurt it without their weapons or bodies starting to melt themselves.
- The Amethyst twins getting caught in a "crane" trap in the beginning of Volume 3, made worse by the fact that the spikes are actually pieces of Sapphire, a long-lost comrade. In the anime, Kanae Itou's acting takes this scene to the next level, the Amethysts still speaking in their typical mellow voices as they helplessly beg Phos to run, before letting out a bloodcurdling scream.
- Phos suffers a fair degree of Sanity Slippage throughout the story, with two incidents standing out in particular. First, while out breaking ice floes with Antarctite, they starts lamenting how weak their original arms are in comparison to their new legs; the ice floes can make sounds that resemble speech to the gems, and Phos ends up hallucinating that the ice is encouraging them to have an 'accident' to get another transplant. "But you could become strong fast." "I'll bite it off for you." "You need to change." And they almost goes along with sticking their hand into a gap between the floes! Later, when the Lunarians attack while Phos is patrolling with Cairngorm, Phos - with little to no warning - attacks Cairngorm due to confusing them with Antarc, who was shattered. "It's dangerous. You may be taken away, Antarc." And then they begin calling other gems Antarc as well. Phos's expression during all of this actually makes it look like they're having a bout of PTSD.
- The gold-and-platinum alloy Phos obtains has a mind of its own at first, trapping them in a cube and seemingly trying to smother them while Antarc is shattered and taken away by the Lunarians. And during the above-mentioned moment with Cairngorm, not only does Phos trap them in a replica of this cube, but the alloy ends up shattering Phos's face into some kind of...warped flower thing.
- The humanoid figure that the Lunarians suddenly drop in front of the gems - it doesn't talk and seems to have a flowering growth where its face should be. It's actually a failed clone of Adamant's creator.
- What gems look like when they are just born. Stiff, eyeless statues vaguely humanoid in shape until Sensei carves them and inserts new eyes himself into them.
- Cairngorm's deep rooted insecurities causing them to crack from the ground up. The whole scene is unnerving: after learning that their devotion to Phos may just be Ghost's inclusions controlling them, Ghost's, well, ghost, drapes over Cairngorm while they scream that they want to be free. The gem leaves marks everywhere, scratching the surface of the floor and part of the wall, and later can seen crawling by the time they calm down enough to accept Aechmea's help.
- There's an eerie sort of feeling when you look at the contents of the second art book. All of the Gems are depicted as busts, either with some sort of deformity (like the Amethysts being Conjoined Twins at the face or Lapis being smushed against the ground) or just standing up with a serene, but dead expression (i.e. Pasparascha, Yellow Diamond), each looking like they're propped up as actual jewlery. Then you get to Phos, who has a Thousand-Yard Stare and is barely balancing on pieces of their replacement materials, like the gem's about to snap. Their head is also the only one that looks like it doesn't interact with the audience at all, which is fitting, given that this is Phos' original decapitated head.
- Chapter 79: After suffering centuries of indignation after indignation, Phos finally snaps and in a display of mineral-based Body Horror, takes out their wrath on the one they perceive as the source of their misery, Adamant, all while roaring for the latter to keep on praying and end it all.
- Chapter 80: The Prince reveals that Adamant's praying will not only affect the Lunarians, but everything of human origins, including the Gems.
- Chapter 82: Phos, upon realizing that the Lunarians were keeping things from them about Sensei's praying, declares that they will turn every gem into dust until they're free. The monstrous form they takes shape of as they prepare to strike is haunting, to say the least.
Nightmare Fuel / Land of the Lustrous