Funny you should post this today...I actually just posted something about zincite yesterday on the cloudy tananite thread...it was a "real/fake mineral" that was trying to be passed off around the min' world a few years ago...haven't seen cut ones really before...thanks for the link sonoma
Sphalerite is Zinc sulfide, so I think this is a closely related gem species. They both look very similar, and they are both the same hardness (very soft). I guess most, not all, Zincite is an accidental byproduct of man though (synthetically made).