The Shining Mirror
Obsidian is formed by laval that cools too fast for minerals to form. It is a naturally occuring glass. Its cool, opaque blackness is used to make magic mirrors, and one may perform divination even by gazing into the stone itself.
It's a stone of earth, of grounding, a portal to the hidden realms, a revealer of secrets, a truth-telling mirror. Whisper your deepest secrets into its depths, and it will keep them safe. Imbue it with your desires and it carries them into the cauldron of transformation to become real.
About 2.5" x 2.5"