Large Amethyst

Large Gems exist for Capture the Gem, the informal PvP mode where each team guards a colored gem and tries to haul the enemy one home. That explains behavior nothing else in your inventory has. Carry one and it floats over your head as a hologram everyone can see, carry several and they orbit you, and it drops on death no matter which character difficulty you picked. Useful in a capture game, awkward everywhere else.

The hologram is bright enough to light a dark cave, which is a pleasant accident. Less pleasant: putting one in your trash slot destroys it instantly with no way back, so be careful when you are clearing space in a hurry.

Two things make one worth keeping in single player. It fits a Gem Lock as a key, which is how adventure map builders gate a door or fire off an event. And an anvil will fuse it to a Minecart for a gem cart that rides exactly like the plain one but sparkles. Large Amber is the odd member of the family, with no team color behind it and a sprite that ignores the shape the rest follow.