This article's content is accurate to the most recent public game version, 0.4.8d.
"A strange bauble of tremendous power."
The Black Gem is a powerful consumable item that fully restores the HP/MP/TP of the entire party.
Only one Black Gem is available in the game. It can be acquired at the end of Rania's Fun House by interacting with the cracked mirror and choosing not to fix or change the party's names. Given the nature of name changes affecting all saves, it is possible to create another save at the location to use to change the names while still getting the Black Gem on the file you are playing on.