Editors

Precious and imperial topaz from Brazil

EditorsJan 25, 2013

Precious and imperial topaz from Brazil

Brazil produces more topaz than any other country in the world. Most of the production is colorless, which is irradiated and heated into various shades of blue. Precious and imperial topaz offer a rare and warmer alternative to the cool blue shades we are accustomed to seeing in jewelry stores. “Precious” topaz usually refers to yellow to orange colors while “imperial” topaz usually refers to pinkish-orange to pink to purple and even red colors (see figs. 1 and 6).  However, some dealers argue that all topaz in these warmer hues should be classified as imperial. Since the origin of the word topaz is most likely from the Sanskrit word “tapas,” meaning fire, this argument seems to have merit. Recent price increases are due to limited mining, renewed interest from designers in these warmer hues and strong domestic demand in Brazil.



Fig.1: Precious and imperial topaz from Ouro Prêto, Brazil (Gems from 5 carats to more than 62 carats)
Center: Classic orthorhombic crystal with prismatic termination (102g). Photo: E. Boehm, RareSource

No visit to Brazil would be complete without stopping by the famous gold mining region of Ouro Prêto, “Black Gold” in reference to the black rock in which gold was first discovered there during the Brazilian gold rush of the early 18th century. Today, this beautifully preserved colonial Portuguese city is a UNESCO World Heritage Site and the world’s only source for imperial topaz.

Ouro Prêto, while built on gold mining, is today more famous for precious and imperial topaz, which was also discovered here in the early 1700s. The topaz mines are located just a short drive outside the city so it is possible to visit them in a few days. Getting there is easier if one flies through the closest major airport in Belo Horizonte, which is only a two- to three-hour drive from Ouro Prêto.

The two main large-scale mines of Capão and Vermelhão produce the majority of all material on the market today but there are also several small-scale mines operated by independent artisanal miners known as Garimpeiros. Vermelhão is the Portuguese word for vermilion in reference to the reddish-orange to red topaz crystals found there. All the mines are open-pit operations that use water to separate the gem crystals from the weathered host rock.


                              

Fig.2: Washing grate (top left) used for the initial separation of host rock and gem-bearing ore, which flows down the slope to the rolling jig that further separates the gems from the ore. Capão Imperial Topaz Mine, Brazil.
Photo: E. Boehm, RareSource

Fig.3: Fine topaz crystals (top right) displaying the orange to sherry range of color found at the Capão mine in Ouro Prêto, Brazil. Photo: E. Boehm, RareSource


Fig.4: Topaz crystals ranging from golden yellow to vivid pink. Photo: E. Boehm, RareSource

Geology
Topaz is an alumina-silicate that typically forms in granitic pegmatites or rhyolitic igneous rocks.  Imagine a molten volcanic material that travels from the lithosphere deep in the earth through the asthenosphere and ultimately into the Earth’s crust. This molten material either migrates up as a large plutonic mass, or squeezes into weaker fractured areas within the earth’s crust creating pegmatites, or explodes out in the form of a volcanic rhyolite. Once it
hits the solid crust or is exposed to the surface, it cools and crystallizes. There are two parts to this crystallization process. The first is the pegmatitic phase, which may also involve the mixing of the surrounding crust’s minerals with the molten intrusive pegmatite. The second is the hydrothermal phase, which involves the introduction of mineral-rich fluids into the pegmatite. This is a very basic explanation of the extremely complex nature of how petrological forces effect the formation of various pegmatite minerals such as topaz.



Fig.5: Topaz crystals in host rock from the Vermelhão Mine in the Saramenha district of Ouro Prêto within the state of Minas Gerais, Brazil. Photo: E. Boehm, RareSource

Gemology
The basic chemical formula for topaz is Al2(SiO4)(F,OH)2.  Precious and imperial topaz gets its colors from trace amounts of chromium (Cr+3). It has a relatively high hardness of eight on the Mohs scale, which is the same as spinel. Like diamond, it has one direction of perfect cleavage so care must be taken to avoid hard blows. Its relatively high refractive index (1.61-1.63) and excellent transparency make topaz one of the most versatile and desirable gems. Like tanzanite and chrysoberyl, topaz is no-axial which means it has two optic axes and three optical directions through which light travels at different speeds. These three optical directions represent the three refraction indices of topaz. This also means that each direction has a different absorption producing different colors. Blending these colors in the faceting process is a true art and can produce amazingly varied results, particularly in precious and imperial topaz. Topaz typically contains two-phase inclusions (liquid and gas), three-phase inclusions (crystal, liquid and gas), or multi-phase inclusions (crystals, liquids and gas). These multi-phase inclusions are a result of the hydrothermal phase during which crystal growth occurs.


Fig.6: An extremely rare 17-carat imperial red topaz, square cushion, Ouro Prêto, Brazil Gem Courtesy RareSource, Photo by R. Weldon

Graduate Gemologist (G.G.) Edward Boehm is the owner of RareSource (formerly JOEB Enterprises), a Chattanooga, Tenn.-based gemstone supply and consultancy. RareSource travels to mines worldwide to bring customers the finest quality gemstones. Contact him at edward@raresource.com. For those attending the Tucson show next month, please feel free to drop by the RareSource booth #406 at the AGTA show.
Edward Boehmis a Graduate Gemologist and the owner of colored gemstone and supply consultancy RareSource.

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