It's a green crystal, similar to Emerald but with special qualities. It is Pure Crystallized Mental Energy. We could also call it "Grialic Crystal" since the Holy Grail would also be made of this same material.
These Crystals have incredible power. They are intelligent, and can be programmed like a computer for multiple purposes, depending on how you carve them. In the case of Retention Crystals, they are programmed to trap Essences and fix them on a Planet.
They were created by the Extraterrestrials of higher dimensions.