Monday, 25 November 2013

Gemstones of Minas Gerais, Brazil

Brazil produces the greatest variety of gems and semi-precious stones in the world, including diamonds, emeralds and amethyst.
Portuguese colonists created the state of Minas dos Matos Gerais, later renamed "Minas Gerais," in 1720, while exploring this region of Brazil in search of gems, gold, and other valuable natural resources.

Some of the most spectacular gems come from the state of Minas Gerais. This area produces emerald, aquamarine, rubellite (pink tourmaline), green tourmaline, imperial topaz, alexandrite and amazonite.
During the 1700s, when Minas Gerais was under Portuguese control, gold and gem mining labor was conducted by slaves taken from the west coast of Africa. This accounts for the large percentage of African descendants who now populate the region.
The gem-bearing pegmatites of the Minas Gerais region are some of the most significant deposits in Brazil and the world. The topography consists of hilly country defined by mounds called inselbergs, which are pegmatites that eroded more slowly than the surrounding schists and metamorphic rock.

As the pegmatites began to erode after prolonged exposure, tougher material such as beryl, topaz, tourmaline and quartz were transported to secondary eluvial and alluvial deposits located in downstream drainage basins.
The finest Brazilian aquamarine comes from the Marambaia valley region of Minas Gerais, 100 kilometers north of Teófilo Otoni. The highest concentration of aquamarine occurs with gem-bearing pegmatites located near the villages of Catugi, Ponto de Marambaia, and Padre Paraíso.
Topaz mining in the vicintity of Ouro Prêto dates back to the 1700s. Imperial Topaz was originally mined in the "topaz belt" of Minas Gerais, which was comprised of three distinct regions (Dom Bosco, Rodrigo Silva, Saramenha) near the mining town of Ouro Prêto. In 1768, he Portuguese government designated all gem deposits in the region as property of the crown, under state control. The Mina do Capão, near Ouro Prêto has produced some of the finest imperial topaz (topázio imperial) in Brazil.
Brazilian diamond mining in Minas Gerais was short lived. In 1714, Brazil's first diamond discovery was made by miners prospecting for gold along the Rio Jequitinhonha river near the town of Tejuco (now Diamantina), in the Espinhaço Mountains.
By 1729, the Portuguese government had declared the placer mines as the property of the crown, labeling the area as a "forbidden district," and proclaiming a "royal monopoly" on all mining.

In the years that followed, there was a diamond-rush to the Rio Jequitinhonha and Rio Abaeté alluvial deposits, and diamond production in the region reached its peak between 1785 and 1807. Thereafter, there was a rapid decline as the area became mined out. The largest red diamond known, the "Moussaieff Red" was discovered in the 1990s by a Brazilian farmer in the Abaetezinho river in 1990. The rough stone weighed 13.9 carats (2.78 g).
Brazil is the one of the largest volume producers of emerald in the world, and the states of Minas Gerais, Bahia, and Goias have the largest deposits in the country. Significant emerald deposits in Minas Gerais are found at the Capoeirana mine and Itabira mine in Nova Era, Município Itabira, about 90 kilometers east of Belo Horizonte. Brazil has been known to produce large rough emeralds up to 200 carats in size.
Brazilian tourmaline is found in igneous rock pegmatite dikes and alluvial deposits (cascalho), located in the area around the Município de Virgem da Lapa, north of Teófilo Otoni. The Ouro Fino, Arazuai-Itinga, and Arazuai-Salinas mining areas north of Marambaia are a significant source for green (chrome) tourmaline.