When I was looking at buying the GDLP I talked with Harrelson about the differences between the cast iron and the glass plates. I was concerned that if I were to drop the glass it would break, He said that if you drop the iron it could go out of whack. The stone lapping side on the cast is diamond just like the glass. Here is the cast plate described on the web. I think the powder is for the iron side to true chisels. He says never to use it on the diamond side.