Snowflake obsidian

Snowflake Obsidian was a variety of obsidian found in the Realms in large deposits (of volcanic origin).

Description
This ornamental stone was black with grayish, flower-like patches that often, especially if some stone could be cut away, radiated symmetrically, resembling snowflakes. It was brittle and weak, but often used for small carvings. It was either tumbled to gemstone form for sale, or sold in "trade blocks" (large, irregular chunks, as quarried) of up to twenty five pounds (twelve kilograms). A typical stone had a base value of 10 gp.