Obsidian is volcanic glass, considered a rock rather than a mineral.  It is formed in the last stage of basaltic eruptions, when the silicas, left over after most other elements have been used up, are ejected and chilled at surface temperatures.  Rainbow obsidian has a multicolored iridescence.

Shells are mineral structures formed by aquatic creatures and land mollusks. Lustrous shells including mother-of-pearl have been carved into cameos and buttons since antiquity. The beauty and variety of shells make them a sought-after decorative material.