Source: Dungeon Master's Guide

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.

Gem Summoned Elemental
Blue sapphire Air elemental
Yellow diamond Earth elemental
Red corundum Fire elemental
Emerald Water elemental