Introduction: Danburite is calcium borosilicate (CaB2(SiO4)2) named after Danbury, Fairfield County, Connecticut. It was initially discovered in 1839 by Charles Upham Shephard, a prominent mineralogist and assistant to Benjamin Silliman. Silliman was a professor of chemistry and natural history at Yale University and the founder of the American Journal of Science. The mineral sillimanite is named after him. It is interesting to note that none of the material from Danbury was ever large enough or gemmy enough to be faceted. The original locality has long been covered over during the build up of the city over the years. As depicted in the photo to the left, the danburite crystal is orthorhombic and very similar to the habit of topaz crystals. Danburite has the durability to be an acceptable gem for use in jewelry and also has become a favorite among the metaphysical crowd in recent years.
Colors: Mostly colorless, also yellow and light brown. Pink is sometimes seen in Mexican crystals when viewed against a stark white background. The pink color virtually disappears during cutting and is not seen in the finished stones even with a good imagination.
Localities: Mexico, Tanzania, Madagascar, Burma, Russia and Japan. Seen in the photo above from left to right are 3.12 ct cushion from Madagascar, 3.04 ct trillion from Russia, 14.89 ct radiant from Mexico, 3.35 ct cushion and 1.84 ct oval from Tanzania.
Availability: For many years, colorless danburite from Mexico was plentiful in the market but supply has dwindled in recent years. Sporadic amounts of light brown and light yellow danburite are seen from Madagascar and Burma. Russia is a source for colorless danburite, but also for a richer brown material that is reportedly irradiated. The material from Japan is in very limited quantity. Recently, yellow danburite from Tanzania has been found. Although there has been quite a bit of material on the market in the last several years, recent supply has been limited and prices are on the rise.
Clarity: Colorless and brown danburites are usually eye clean and sometimes even loupe clean. While eye clean material is always desired, yellow material is often seen with needle-like inclusions that seem to be directly related to depth of color. In other words, the more intense the yellow, the more the needle like inclusions.
Stone Sizes: Although large danburites have been cut in the past, including a 138 ct gem from Burma, most of the available danburites in the market today are less than 20 ct.
Treatments: Almost all danburite is natural and unenhanced with the exception of the dark brown material from Russia which is reportedly irradiated.
Properties: With a hardness of 7, no cleavage to worry about, and overall “good” toughness, danburite makes for a fine gem to use in jewelry or a collection. The refractive index is not overly high (1.630-1.636) and the dispersion is very low (0.016) but stones take a high polish, so well cut gems are surprisingly bright. The optical properties are similar to that of topaz so care must be taken to accurately identify a gem set in jewelry. Unset gems are easily separated from topaz based on specific gravity. The use of ultrasonic and steam cleaners would be considered risky and best avoided.
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