If you do go with glass, you might be able to use some borosilicate (lab glass, very tough & clear). I'd suggest that you could troll for a broken smart phone and use the glass from the screen, which tends to be really tough, but I'm not sure how you'd cut/grind it - out of my area of expertise, unfortunately . . . .
Bullseye glass, which is still soda-lime glass, is tougher than a lot of other soda-lime, and in theory could be used to make a cabochon, but you're going to end up with some waves/texture on the back from the kiln shelf, pretty much no matter how smooth you put the kiln wash on - not anything too serious, but you can pick it up in a couple items I've got in my shop here:
https://www.etsy.com/listing/110340268/fused-glass-pendant-clear-pink-lavenderhttps://www.etsy.com/listing/110340268/fused-glass-pendant-clear-pink-lavenderYou'll have to pardon the horrid pictures, I haven't had a chance to re-take them, and now that I see what's wrong with them, they're driving me nuts ;)