Agatized Coral maybe? This one is thin and I thought I was going to lose it several times. But since I've never seen another one like it, I'm putting this one in the collection.

24mm x 3.5mm.   Weight: 17.30 carats.

Click the image on the right to see the cabochon super-sized...

This cabochon is in my personal collection. It is not for sale.