Obsidian's origin is different from the most other natural stones, slowly built by mineral crystals. Obsidian is born in explosions, fire and lava, it is a volcanic glass without any crystal structure.
Obsidian is mostly opaque black, but mahodany obsidian also has a lot of brown in it, displaying beautiful swirls and stripes of color.

Note: the image gives you a good general idea of what the pendant looks like, but colors and patterns of natural stones may vary from lot to lot.