Amethyst is a stone that is highly valued in that it differs from common quartz because of its violet to purple color. The coloration is caused by the presence of iron and/or manganese. Amethyst has an absorption spectrum of 550-520 and its color can be restored or enhanced by the high energy of X-ray radiation. As a member of the quartz group, it has a hardness of 7, density of 2.65 and streaks white. It occurs in the trigonal crystal system and is composed of silicon dioxide. Amethyst are found in geodes of alluvial deposits. The most significant deposits are in Brazil, Madagascar, Zambia, Uruguay and many others including the United States (Arizona). It is a birthstone for February and is commonly used in jewelry. Ametrine or tristine is half amethyst (purple) and half citrine (yellow) and its deposits are located in Brazil and Bolivia.