Opals (and not-opals)
Black opal and white opal are exactly what they sound like: opals with a black or white base color!
Crystal opal is a nearly transparent opal with vibrant play-of-color throughout the stone.
Fire opal can be red, orange, or yellow, and may or may not have play-of-color.
Boulder opal is characterized by its matrix, usually a rich brown rock surrounding blueish opal with vibrant play-of-color!
Synthetic opal (forgive my drawing lol) can usually be recognized by its too-even, scale-like play-of-color on one surface, and columnar/stripey play-of-color at right angles to that surface. It also often comes in unnatural body colors like pink, bright blue, green, etc.
Opalite isn't even related to opal! It's just a type of glass. It's always the same light blue with orange in transmitted light.
Bonus: a bunch of different play-of-color patterns are illustrated here. The boulder opal shows pinfire, the crystal opal shows flashfire, and the black opal has areas of harlequin, the most valuable pattern! The others are somewhere in between, like most real opals.
