Opal Stone – Natural, Cabochon, Black

This Andamooka matrix opal certainly resembles fine black opal. However, if you look closely at the black areas you'll see this matrix opal received a sugar-acid treatment. "Andamooka matrix opal," photo by Dpulitzer. Licensed under CC By-SA 3.0.

Article: Opal Treatments: How to Detect Treated Opals

Credit: Dpulitzer