Snowflake obsidian

Snowflake Obsidian was a variety of obsidian found in large deposits of volcanic origin.

Description
This ornamental stone was black volcanic glass with grayish splashes in the matrix that often exhibited snowflake-like symmetry if cut correctly. It was brittle and weak, but could be used for small carvings. It was either tumbled and sold as a gemstone, or quarried in "trade blocks" (large, irregular chunks) of up to 25 lb (12 kg) each. A typical gemstone had a base value of 10 gp.

Powers
Snowflake obsidian could mitigate damage to creatures or objects from dragon's breath. A piece of this gemstone at least the size of a thumbnail had to be worn on the outside of armor or mounted on the outer surface of an object. When exposed to dragon's breath, the stone absorbed an unpredictable portion of the damage and disintegrated.