ISLAND (or Iceland) Crystal, a body famous among the writers on optics, for its property of a double refraction; but named very improperly, as it has none of the distinguishing properties of crystal, and is plainly a body of another class. Dr Hill has determined it to be a genus of spars, which he has called, from their figure, parallelopipedia, and of which he has described several species; all of which, as well as some other bodies of a different genus, have the same properties. It is always found in form of an oblique parallelopiped with six sides; and is found of various sizes, from a quarter of an inch to three inches or more in diameter. It is pellucid, and not much less bright than the purest crystal; and its planes are all tolerably smooth, tho', when nicely viewed, they are found to be waved with crooked lines made by the edges of imperfect plates. What appears very singular in the structure of this body is, that all the surfaces are placed in the same manner, and consequently it will split off into thin plates either horizontally or perpendicularly; but this is found, on a microscope examination, to be owing to the regularity of figure, smoothness of surface, and niceness of joining, in the small parallelopiped concretions of which the whole is composed. It is very soft, and easily scratched with the point of a pin. It will not give fire on being struck against steel; and ferments and is totally dissolved in aqua fortis. It was first discovered in Iceland, from whence it has its name; but has been found in France, Germany, and many other places. In England, fragments of other spars are often mistaken for it; many of them having in some degree the same properties. For an account of the singular refractive property of this substance, see (the Index subjoined to) OPTICS.