Talk:Obsidian/@comment-37209390-20190127111019/@comment-5642360-20190131135311

The iridescence you see on the gem is caused by a layer of contaminants that harden on the surface as the mineral cools. Obsidian is more or less just black. The iridescence can also be seen on laboratory/home made Bismuth crystals. The actual metal is just gray, but if you cool it in a controlled environment, it'll stain with multiple colors.