Obsidian is "volcanic glass"composed of silica based magma that had little time to cool resulting in no crystalization. Occasional impurities will sometimes begin to crystalize resulting in various effects as in Rainbow Obsidian and also "gold sheen" obsidian.
Also in the obsidian family are "Mahoganey"; which is black with striking brown streaks and "Apache Tears" which are dark almost black yet quite clear.
Check currency conversion