The distinction between a mere mineral and a gemstone is not really 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 nonetheless a number of interesting minerals which are quite soft but which are of great interest to gemstone collectors. A good example is cinnabar. It is not only a colorful mineral, but it has some quite unusual gemological properties.
Cinnabar is mercury sulphide by composition and is the common ore of mercury. Its typical color is a brick-red to cinnamon red to 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. But it is also remarkably dense, with a specific gravity of 8.1. By way of comparison, the most dense gemstones commonly sold are spessartite garnet (4.12-4.18), zircon (3.93-4.73), sapphire (3.95-4.03) and ruby (3.97-4.05). Hematite, a less common gemstone, has a density of 5.12-5.28.
In fact there are only a few minerals of any sort that are denser than cinnabar. These minerals tend to be metals, such as gold (15.5-19.3) and silver (9.6-12.0). For a non-metallic mineral cinnabar is unusually heavy.
Cinnabar is not only remarkably dense, it also has an extremely high refractive index (2.905-3.256), much higher than that of diamond (2.417-2.419), zircon (1.810-2.024), demantoid garnet (1.88-1.94) or sapphire (1.762-1.778). Given these unique properties it is not surprising that cinnabar is popular with collectors.
Cinnbar forms in trigonal crystals. But individual large well-formed crystals are rare. Usually cinnabar is found in crusts or crystal complexes, and massive forms are common. Some fine rare crystals have been found in Spain and China.
Notable occurrences of cinnabar are in Almaden, Spain; Idrija, Slovenia; Hunan Province in China; and California, Oregon, Texas, and Arkansas, USA.
- First Published: February-04-2010
- Last Updated: November-07-2013
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