I'll leave some of the coral whole, Joe just to try it.
Scott, I got this from Cowboy years ago and it has worked for me until I got a kiln. A twenty # bag of charcoal all night seems to work really good for the lesser cherts and coral. We do have some coral that has to be heated at higher Temp's but it doesn't knap as well to me.
This is the way he has done it or his Texas rock.