Topaz is aluminum fluosilicate, and is common and abundant in many parts of the world. It occurs worldwide as fine crystals, often transparent, some weighing as much as 100 kilograms, and is found in a wide variety of colors. Crystals are often incredibly sharp and well formed. Topaz is also an important gemstone and represents the month of November. Precious topaz is orange material from Brazil; Imperial topaz is reddish-orange, red or pinkish-orange in hue. Blue topaz is produced in huge quantities by heating colorless topaz that has been turned brown by irradiation. Fine natural blue topaz crystals occur at Mursink, Siberia, Russia. Careful laboratory tests are required to distinguish natural from irradiated topaz.