I think you're thinking of another stone....
or maybe the big fakes that have been around forever....I've been a collector for a long time, and I've seen lots of the big fakes (I've got a nice doorknob) but finding a real one of decent size and colorchange is damn near impossible.
Real alexandrite has never been plentiful and it's never been cheap. Seriously, I have a tiny quarter carat that I got years ago and it was appraised as authentic.
Maybe you're thinking of the color change topazs or garnets?
I don't know what they sell for today ( which is patently obvious by my previous posts ), but I certainly DO know what they were going for several decades ago, in Bermuda.
Garnets look NOTHING at all like alexandrites. Garnet was my great grandmothetr's birthstone and I grew up being allowed to wear them, when I played dress-up. Garnets and alexandrites are semiprecious stones, but garnets do NOT change color.
If you want to discuss this further, I suggest that we do it in private, since the topic of this thread is diamonds. :-)