Yikes!
Was I ever disappointed by this! When I saw the word "encyclopedia" I assumed that's what it was, and that it was going to be a good one. But the information within is terribly scarce. The author seems to have picked only his favorites to include, when really all stones and metals should (In my opinion, even if they're not expensive or popular) be covered at least in passing, and left out a variety of information about the materials themselves and instead talked about just the more common magical uses and folklore. Anyone looking for information on stones, crystals, and the like can get better than this.