Garnet is one of the three gemstones known under the ancient classification of carbuncle. Garnets appear in several types, including pryope (blood red) and almandine (purple-red). Often these two types appear together in the same stone. Among the lesser known types are demantoid and Tsavorite, both of which are a vivid green color. These latter two types were unknown in the Middle Ages. Tsavorite was only discovered in 1974.
Garnet belongs to a group of aluminum silicates—Al2(SiO4)3; the coloring comes from several differnt metals, such as magnesium in pyrope—Mg3Al2(SiO4)3 or iron in almandine—Fe3Al2(SiO4)3. Most red garnet is rhodolite, a combination of almandine and pyrope. Garnet's hardness ranges from 6½ to 7½ on the Mohs' scale. Both almandine and pyrope are harder than quartz and very durable, making them suitable for all types of jewelry.
Walter Schumann, Gemstones of the World, New York, 1997.