Wondrous Item, uncommon
This yellow diamond contains a mote of elemental energy. When you use an action to break the gem, an earth elemental is summoned as if you had cast the conjure elemental spell, and the gem's magic is lost.
Notes: Summoning
Does the use of the Gem, like the spell itself, require concentration. I would think not because your unleashing the element, not calling it forth.
How long does the Elemental Last
1 Hour, as per the spell.
It's summoned as if you used the spell conjure elemental so it requires concentration.