Gorgeous olive or sage green mica or slate stone earrings in gold square w/backs

I can't find any stone like these anywhere on the internet to know what exactly they are. I describe them as olive green slate in gold earrings with backs. When the light hits them a certain way, they display a rainbow of colors. so beautiful. One pair, worn only a few times many years ago (probably in late 1990's or around there). They were a favorite of mine that I purchased to go with a business suit of the same color. I think that's the only outfit I wore with them. The backs are posts with the larger plastic circle to secure them in place as you can wear them as a square or as a diamond shape. I wore those style of backs more often as I had one piercing that was a little off. approximately 3/4" square They have been cleaned in gentle jewelry cleaner and packaged as shown.