Black opal

Black Opal is a greenish type of opal with black mottling and gold flecks. Usually found in ancient hot springs, the gem is usually tumbled smooth and cut cabochon. The phrase in the North "Black as a black opal" means, effectively, not very black (or evil) at all, and is used to describe good-hearted rogues and similar individuals who would be embarrassed by praise. A typical specimen has a base value of 1000gp.