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Opal is typically categorized based upon its body color – light or dark, as well as the degree of transparency. The body color doesn’t refer to the play of light, but to the actual color of the stone itself, which can be colorless, white, various shades of grey, or black.
Light opal includes all the shades through medium grey
Dark opal includes dark grey to black
In addition, opal can have varying degrees of transparancy, from transparent to translucent to opaque. When an opal is transparent or very translucent, and the clarity of the color is very sharp – the play of color must be both on the surface and within the stone – the opal is called crystal.
Jelly opal is a type of crystal opal that has pla of light within the stone that is hazy, rather than clear.