The name opal originates from the Greek word “Opallios”, which meant to see a change in color. Opal contains up to 20% water trapped in its silica structure. This combined with hardness of only 5.5 to 6 on the Mohs scale, makes opal a delicate gem that can crack or craze under extreme temperature, dehydration or direct light. Because opal has the colors of other gems appearing in its sheen, the Romans thought it was the most precious and powerful of all. Opal is the birthstone for October and also the gemstone that celebrates 12th and 14th wedding anniversaries.
- White or light opal: Translucent to semi-translucent, with play-of-color against a white or light gray background color, called body-color.
- Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
- Fire opal: Transparent to translucent, with brown, yellow, orange, or red body-color. This material—which often doesn’t show play-of-color—is also known as Mexican opal.
- Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
- Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.