It is projective and is used for grounding and divination. Obsidian is naturally occurring glass: it is simply lava that cooled so fast that the minerals contained within didn't have time to form. The ancient Aztecs fashioned it into mirrors. It also was popular for fashioning stone knives and spear heads. In Mexico obsidian is still fashioned for scrying tools. For
some, the blackness of the stone allows easier contact with the subconscious mind.