not to hyjack the thread but i got some nice sedimentry stone the other say it was a mud stone and it gives the best finish on my knives like using a coarse stone because it still has bumps and needs to be lapped but finishes like a weird natural. i was told it was a mudstone that had been pressurised over the years to make it so hard and also some petrified wood stone. needs to be lapped to be used