Wooden Coasters!

I think it might be black walnut that's not mature?

 I found some beautiful wood at the side of the road a few weeks ago, out for garbage pickup from someone pruning their trees. It's pretty hard to find nice fresh wood that isn't yet riddled with bugs, and still in whole logs and not chopped up like firewood. So with some help I brought a few good-sized ones home. A few days ago I started sawing them into slices, and from one log, I got some good sized coasters! The only thing is I'm scared about them cracking. They're new wood and haven't shrunk yet, and I don't have access to a  drying kiln. Hopefully they wont shrink/crack that much. I also start wood shop in a few days!  I'll finally be able to find out what I should do to prevent cracking!