I usually say emerald is my favorite, although I think my true favorite is probably malachite. Black opal might take over my top spot sometime soon though.
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@SW-User Ruby is a crystalline form of aluminum oxide which contains traces of chromium. It's the chromium which gives ruby its beautiful color.
SW-User
@helenS yeah, the d-d transition of electron of octahedral sites occupied by chromium in its +3 oxidation state gives the rise to the colour. Magnificent.
