Obsidian gemstones, tumbled and polished.  Obsidian is a natural glass of volcanic origin and has a glassy lustre and is slightly harder than window glass. The color is a rich jet black, and typically used as a semiprecious stone. 

Gemstones and mosaics:  Tumbled gemstones make colorful and interesting accents for "crazy-quilt" designs in mosaic backsplashes, picture frames and tables. The stone adds a little texture to an otherwise perfectly flat surface. 

Sold by weight, not by piece count.  Keep in mind you are buying a natural material, and stones may vary from the picture shown.  No two stones are exactly alike!