Red Cinnabar Gemstones
The distinction between a mineral and a gemstone is not always well-defined. In general we can say that an attractive mineral will find use as a gemstone only if it is sufficiently hard and durable to be set in jewelry.
Cinnabar Crystals from Spain
Having said that, there are a number of interesting minerals that are quite soft but attract gemstone collectors nonetheless. A good example is cinnabar. It is not only a colorful mineral, but it also has some quite unusual gemological properties.
Cinnabar is composed of mercury sulphide and is the common ore of mercury. Its typical color is brick-red, cinnamon red or bright scarlet. Generally cinnabar occurs as a vein-filling mineral associated with recent volcanic activity or alkaline hot springs. Cinnabar was known to the ancient Romans both as a pigment and a source of mercury. It was also used for decorative purposes in South America and China.
Cinnabar is notably soft, with a hardness of only 2 to 2.5 on the Mohs scale. However, it is remarkably dense, with a specific gravity of 8.1. By way of comparison, the most dense gemstones commonly sold are spessartite garnet (which has a specific gravity of 4.12 to 4.18), zircon (3.93 to 4.73), sapphire (3.95 to 4.03) and ruby (3.97 to 4.05). Hematite, a less common gemstone, has a density of 5.12 to 5.28.
Indeed, there are only a few minerals of any sort that are denser than cinnabar. These minerals tend to be metals, such as gold (with a density of 15.5 to 19.3) and silver (9.6 to 12.0). For a non-metallic mineral, cinnabar is unusually heavy.
Cinnabar is not only remarkably dense, it also has an extremely high refractive index (2.905 to 3.256), much higher than that of diamond (2.417 to 2.419), zircon (1.810 to 2.024), demantoid garnet (1.88 to 1.94) or sapphire (1.762 to 1.778). Given these unique properties it is not surprising that cinnabar is popular with collectors.
Cinnabar forms as trigonal crystals, but large, individual well-formed crystals are rare. Usually cinnabar is found in crusts or crystal complexes and massive forms are common. Some rare, fine crystals have been found in Spain and China.
Notable occurrences of cinnabar are in Spain (Almaden), Slovenia (Idrija), China (Hunan Province) the USA (California, Oregon, Texas and Arkansas).