Pseudomorphs are minerals that have been chemically altered, so that they are new minerals that retain the shape of the original mineral. (The word "pseudomorphic" means "false form" in Greek.) Changes in a mineral's environment can cause the replacement of one mineral with another, atom by atom. In one example, limonite, a type of iron oxide hydroxide, substitutes itself in for pyrite, or iron sulfide, but the crystals are still shaped like the original pyrite. Geologists call the resulting mineral "limonite after pyrite." Another example of pseudomorphism occurs when quartz replaces crocidolite, a variety of riebeckite that is a source of asbestos fibers. The resulting solid mineral retains a fibrous internal structure that gives the stone a particular sheen known as chatoyancy, or the "cat's eye" effect.