QUESTION: The mineral books say my crystal should be transparent, but it isn't. Why not? Is my crystal really some other kind of stone?
MY ANSWER: When mineral references state that a crystal or stone should be transparent or translucent, they are referring to a perfect type specimen.
In many if not most examples of any given type of crystal there are inclusions or tiny internal fractures or crazing that make it more opaque. The tanzanite shown is a good example. It is a transparent crystal according to mineralogy but has minute internal crazing which makes it appear opaque. This doesn't mean it's a different type of crystal, just that it has inclusions, internal fracturing or crazing.