Obsidian

Obsidian, also known as natural glass or volcanic glass, came in different grades and varieties, some more prized than others. Low grade obsidian, called pitchstone, was not as shiny or smooth as gem quality obsidian but was useful for grinding and abrading. A variety called blacksmoke obsidian was primarily found in Tashalar, sold in the local bazaars. A rare form of obsidian, called rainbow obsidian, was considered a semi-precious stone.

Description
Volcanic in origin, obsidian was a hard, black, glassy stone with a natural sheen. Common obsidian could be chipped into arrowheads, cutting blades, or even weapons if large enough. As an ornamental stone, it was usually tumbled, polished into smooth cabochons, or carved into figurines and fingerbowls. It was often worn as jewelry or inlaid on bracers or pectorals of copper or bronze. Package handlers in Waterdeep wore obsidian rings with a cutting edge on the inside for snipping twine. Gray to white inclusions made obsidian brittle but when cut and polished correctly, snowflake obsidian was equally as valuable as regular obsidian. A typical stone had a base value of 10 gp.

Powers
There were "nine secrets" that could be enchanted to become ioun stones, and obsidian was the most well known of them. Obsidian also made an extremely useful figurine of wondrous power, renowned as the obsidian steed. The cutting capability of obsidian made it a good material component for wizards attempting to create blade barrier-like effects.