The Star Gem is an Item that was used to get Nuclear Star.
Plays Rain of Amethyst when spawned.
Spawnrate: 1Hr : 1/15
When the item is in your inventory it is simply referred to as "?"
It plays the same music with Crystal Gem and shares a same spawnrate, similar names.
A cutscene used to appear when you used it back then. Now it does show up and kind of destroys your camera. To fix that, just reset.
This item is an reference to Steven Universe.