If I remember correctly, the dredger said that blue clay in California comes from eroded kimberlite, Sometimes diamonds are hard to recognize. The geologist at the Crater park in Arkansas told me once they had a brown diamond was hard to see any of the triangles until under the microscope. It wasn't small but longer and rounded. I've seen photos of diamonds on the Internet with what they call a skin. Some don't look like they have a skin, but large diamonds often need a window cut, at least I read that somewhere.