I'm inclined to agree with Jimmy, and the pages from Dan's Whetstones. They're not just a sales outfit, they own the mines that these rocks come from. Don't get too caught up on color. Novaculite has a variety, and it has the same effect on the hone's performance as that your car's paint color has on horsepower - none. Either it cuts like a Washita or it doesn't. Unless you have a way to test specific gravity, the only way to know for sure is to use them.