Webstone

Webstone was the Realms name for spiderweb obsidian.

Description
This ornamental stone was an obsidian variety formed by small pieces of the stone cemented together by heat and pressure in a globular mass, the joints showing as wispy, web-like lines. It was usually black, the seams sometimes showing white, but brown, reddish brown, and rust-red varieties with lighter-hued web lines were known. A typical stone had a base value of 10 gp.

Powers
This stone offered a wearer protection against airborne pollutants and gasses if worn in direct contact with the skin. Some jewelers fashioned tumbled webstones into armpit bands that were worn around the shoulder to maintain skin contact. When the wearer was exposed to poisonous gas, smoke, or airborne spores, the webstone would vaporize in irregular bursts, protecting the wearer from the harmful effects until the supply was depleted or until the exposure ended. Unfortunately the efficacy of this protection varied greatly&mdash;from complete protection to no protection at all&mdash;between samples of webstone, seemingly at random. This unreliability prevented webstone from having a much higher value.