Elemental gems were naturally-occuring gemstones infused with a mote of Evocation that drew from the Elemental Realms. They could be broken with minor force, which would summon an Elemental from the shattered remains. The type of summoned elemental depended on the gem itself: blue sapphires contained Air Elementals, yellow diamonds contained Earth Elementals, red corundums contained Fire Elementals, and emeralds contained Water Elementals.
The summoned creature was charmed to act as a servant to the bearer, remaining on the same realm as the Spellcaster for one hour before disappearing to their home plane, unless voluntarily dismissed.