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. Their price values are usually less than black opals and sometimes even less than white base opals. Semi-crystal opals are translucent or transparent enough to be able to see a dark thick line through them but not clear enough to read text through them. These opals usually cost less than crystal opals.