Though most people would not recognize this clear mineral as topaz, topaz is usually found either clear or a yellow-brown. The popular blue topaz is not common in nature, but can be made by irradiation and heat treatment of the clear or yellow varieties. The chemical composition of topaz is Al2SiO4(F,OH)2.
Most topaz come from pegmatites. This large crystal capped with lepidolite is naturally blue. This is rare in nature, however, and most blue topaz is man-made by irradiating and heating colorless or yellow-brown topaz. The chemical composition of topaz is Al2 SiO4(F,OH)2.