transparent, coarse-grained variety of the silica mineral quartz (q.v.). Citrine is a semiprecious gem that is valued for its yellow to brownish colour and its resemblance to the rarer topaz. Colloidally suspended hydrous iron oxide gives citrine its colour. Natural citrine is rare compared to amethyst or smoky quartz, both of which are often heated to turn their natural colour into that of citrine. Citrine is often marketed under various names that confuse it with topaz to inflate its price. It may be distinguished from topaz by its inferior hardness and from decolorized amethyst by its lack of reddish cast. Principal localities of occurrence are those of amethyst: Brazil, Uruguay, the Urals, Scotland, and North Carolina. Its physical properties are those of quartz (see silica mineral 【table】).