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.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.