• White or light opal: Translucent to semi-translucent, with play-of-color against a white or light gray background color, called bodycolor.
• 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 bodycolor. 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.

*courtesy of Gemological Institute of America