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.
@SW-User I have an uncut malachite in my jewelry box. It's my favorite piece. I could get it set into a pendant or something but I kinda just like having it the way it is.
@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.
cool question. I probably have a couple favorites emerald is a good one . i like sapphire cus my favorite color is blue. But There are others.. idk if onyx is a gem but it has a cool name.