Calomel is an uncommon mineral found in mercury deposits. It is one of the better-known mercury compounds, and may even be associated together with Native Mercury liquid droplets. Calomel is often found together with Cinnabar, the most common mercury compound, and may form as an alteration product of the Cinnabar.
Calomel derives its name from the Greek term "kalos", beautiful and "melos", black, in reference to its habit of turning black when coming into contact with ammonia.