It’s about the size of a walnut in the shell. I got it with a bag of random rocks and tumbled it through four stages for about a month. The fact that it didn’t get very rounded off means it’s quite hard. It’s more green in real life with lots of neat transparent bits.
Could be, I’m no expert. It’s a lot more green that it appears in the picture, and closest matches I found were agates. Is fossilized coral very hard? It didn’t wear down as fast as the quartz in the same batch.