Species and optical varieties
Alexandrite and cat’s-eye chrysoberyl belong with chrysoberyl; an optical effect does not define a new species.
Keep each chapter’s scientific identity: a family, species, variety, trade label, rock and fossil are different kinds of names.
Mineral species
Chrysoberyl often appears yellow to green and can make a bright, quietly colored gem. Its familiar special varieties can distract from the beauty of ordinary transparent material.
Matteo Chinellato · CC BY-SA 3.0 · Display crop
Mineral variety
Alexandrite is a color-change variety of chrysoberyl. A gem may look greenish in daylight and more reddish under suitable incandescent light.
Original uploader was User: at en.wikipedia · CC BY-SA 3.0 · Display crop
Gem material
Cat’s-eye chrysoberyl shows a narrow band of light across a suitable cabochon. The effect becomes clearer when the stone is moved under a concentrated light source.
Gemshare at English Wikipedia · CC BY-SA 3.0 · Display crop