"Crystal opal" is a term used for a transparent to translucent opal material that has a play of color within the stone.
Crystal opals are often faceted to produce gems that have internal play of color and fire exiting from the facets.
Photos shown here are a crystal opal cabochon with blue to violet play of colors. It is a calibrated 8 x 6 millimeter stone cut from material mined at Lightning Ridge, Australia. Also a faceted Ethiopian crystal opal with a water clear body color and internal play of color. It is four millimeters across.
Find it on Geology.com
More from Geology.com
Ammolite is a fossil and a gemstone. It is shell material from fossil ammonites.
Ant Hill Garnets are tiny garnets that ants haul to the surface and discard on their anthill. Honest!
Strontium Titanate is a man- made diamond simulant with a fire much stronger than diamond.