Elemental Gem

From Dungeon Master's Guide, page 167. Found on Magic Item Table B.

Description
Wondrous item, uncommon.

This gem contains a mote of elemental energy. When you use an action to break the gem, an elemental is summoned as if you had cast the Conjure Elemental spell, and the gem's magic is lost. The type of gem determines the elemental summoned by the spell.