Elemental Gem

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.
GemSummoned Elemental
Blue sapphireAir elemental
Yellow diamondEarth elemental
Red corundumFire elemental
EmeraldWater elemental

Dungeon Master´s Guide (SRD)
  1. DnD 5e Magic Items › Elemental Gem
Elemental Gem