Calcite is a very common and widespread mineral with many forms and colors. Typically it is transparent to opaque and white or colorless, however due to impurities, it can occur in shades of red, orange, yellow, green, blue, violet, brown, gray and even black. Found in most geologic settings in one form or another, it is most common as massive material in sedimentary rocks such as limestone. Because calcite, a calcium carbonate, is relatively fragile and soft (3 on the Mohs scale of hardness), it is not usually an important gem mineral. This large calcite gem from Mexico is a modified step-cut faceted by Michael Gray. It displays the double refraction and dispersion typical of this mineral. It is the largest calcite from Mexico in the National Gem Collection and was donated by Michael and Patricia Gray in 2013.