Despite the abundance of different opal varieties, five opal types take the lead: white/light opal, crystal opal, gray/dark opal, boulder opal and black opal.
Light or white opal refers to any opal with an N7-N9 body tone and can be common or precious. Light opals range from colorless to light gray, while white opals have a white to pale gray body. These opals can be transparent to semi-opaque, but most have a milky tone.
White is the most common opal color and most abundant precious opal type. It’s mined in Australia’s Coober Pedy.
Crystal opal is pure hydrated silica. It does not contain any oxides (the material that turns an Opal white or Black); Crystal Opal is translucent so you can see straight through it when you hold it up to a bright light.
Crystal opals can display any colour of the spectrum.
Primary sources of crystal opals, Coober Pedy, South Australia, and White Cliffs, New South Wales, are producing some of the most beautiful crystal opals.
Gray or dark opal is between white and black, at N5-N6 body tones. Also called “semi-black opals,” these common or precious opals have a medium gray body tone that isn’t dark enough to be black opal. The majority of dark opal comes from Lightning Ridge or Mintabie, Australia. This type is less common than white opal but shows a brighter play-of-color.
Black opal is the rarest opal variety, defined by a dark gray to black body tone from N1-N4. These opals can be transparent black crystal opals or opaque.
Most gem-quality black opals come from Lightning Ridge, Australia. Black precious opal has the brightest play-of-color of any opal.
Boulder opal is a combination of precious opal and host rock, typically ironstone or sandstone, though some contain vegetation or fossilized wood instead of rock.
The precious opal occurs as patches and/or thin seams attached or surrounded by the host rock. This happens when pre-formed opal silica water seeps into the rock’s crevices before the opal hardens.
The main source of boulder opal is Queensland, Australia.