Usually the higher price per carat indicates the flash is more lively and colorful, more evenly distributed, a rarer base color, or have an unusual characteristic, or have any combination thereof. They are generally opaque, translucent, cloudy, and have a rather dull, pale, unattractive color. These opals are plentiful and have very little value. Rarely would we have opals of this nature on our lists. They are the least expensive group of precious opals.
Black opals are much more rarer than white opals and demand a much higher price.
Semi-black opals have a base color that varies from a medium gray to a dark gray but not dark enough to be called a black opal. The price of these opals are usually less than black opals. Gray opals have a base color of medium gray to a lighter gray. The price of these type of stones are usually less than the semi-black opals. Crystal opals are transparent enough to read text through them.