Estimated Value (Sane Cost Guide): 960 gp
DMG Value: 101 gp - 500 gp
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.
Source: DMG p167