The region of pegmatite dikes in the state of Minas Gerais in Brazil has been the primary source of gem beryl and several other species of colored gemstones for many years. Rivers have cut places through the dikes and alluvial deposits, called 'cascalho', which are.
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The region of pegmatite dikes in the state of Minas Gerais in Brazil has been the primary source of gem beryl and several other species of colored gemstones for many years. Rivers have cut places through the dikes and alluvial deposits, called "cascalho", which are scattered throughout the region. It is notable that the extensive gemstone deposits in Minas Gerais often yield large aquamarine crystals accompanied by all the other colored varieties of beryl.
According to Peter Bancroft in Gem and Crystal Treasures, the principal aquamarine-producing region begins about 75 miles north of Rio de Janeiro and includes the areas of "Conselheiro Pena, Governador Valadares, Teofilo Otoni, Aracuai, Salinas, the Jequitinhonha River basin, and Pedra Azul". Numerous sites lie in the Marambaia Valley in the Teofilo Otoni area alone. Many of the locations have not achieved mine status and may be identified only by the name of a nearby fazenda (plantation). Beautiful gem crystals have been recovered from such unlikely places as water wells, drainage ditches, road cuts, and excavations for building foundations.
Most of the largest and finest aquamarines are from Brazil. Imagine being able to see an object distinctly through the completely transparent length of a 19 by 16 inch hexagonal gemstone prism weighing 520,000 carats! Such a crystal was unearthed by David Mussi in the Papamel mine near Marambaia in 1910. Two Germans purchased the 110 kilogram greenish-blue crystal for 35,000 marks. The gems cut from this gigantic crystal were heat-treated in the first known successful application of this method to remove the undesirable yellow tones and achieve a purer blue color. Jaroslav Bauer and Vladimir Bouska wrote in their book, Precious and Semi-precious Stones, that its yield of 200,000 carats of cut gems supplied the world market with faceted aquamarines for several years.
The standard for comparison of color in aquamarine was set by the smaller 34 kilogram, deep blue Marta Rocha crystal recovered from the same area. This very large crystal was cut into 300,000 carats of magnificent gems. Many crystals of another such rich blue tone have been found in the township of Pedra Azul, formerly known as Fortaleza, and the gems of that hue are called "fortaleza" aquamarines.
The Maxixe mine in the Piaui Valley is the source of the notorious dark blue alkali beryl known as maxixe or maxaxite aquamarine. After its discovery in 1917, several lots of the crystals were sent to Germany for cutting. Peter Bancroft says "there was hell to pay" when its color quickly faded to a "whitish yellow". An investigation disclosed that the color was caused by natural radiation and can be restored, but not permanently, by artificial radiation. The crystals soon became quite popular with collectors as specimens, but they were useless as a gemstones. Dr. Joel Arem states in his Color Encyclopedia of Gemstones that maxixe beryl is "rich in cesium". In The Illustrated Encyclopedia of Minerals and Rocks, Dr. J. Kourimsky attributes the color of maxixe beryl to the presence of boron. Dr. John Sinkankas explains in his book, Emerald and Other Beryls, that it is more probable that the replacement of a missing atom by a vibrating electron in the atomic lattice creates a color center which can be destroyed by light and heat. He cites studies by Nassau and Wood to support this conclusion.
Another atypical deposit of aquamarine is located in the Governador Valadares area of Minas Gerais. This site was mentioned in an earlier "Let's Talk Gemstones" article on beryl. The crystals from that site show a pale green body color obscured by multitudes of exsolved ilminite skeleton inclusions. These inclusions form when iron (hematite) and ilminite in solid solution, having been incorporated into the beryl crystal lattice, become supersaturated, are separated at critical pressures and temperatures, and are later expelled from the crystal lattice. They adapt to the cleavage planes of the growing aquamarine and, by repeated recombination and exsolution, create reddish-brown dendritic patterns. When cut properly en cabochon, a six rayed star emerges from the bronze Schiller effect. Similar material is found in Madagascar. These and other dark star beryls can resemble black star sapphires. Cat's-eye aquamarines are very unusual and are seldom available in the market. Examples are shown in the photographs on page 76 of the Eyewitness handbook, Gemstones, by Cally Hall and on page 48 of the softcover version of Dr. Joel Arem's book, Gems and Jewelry.
Aquamarine is almost always found in pegmatites or in the alluvial debris from such sources. Excellent deep blue material is now recovered from a number of mines in Madagascar (the island Republic of Malagasy). Deposits near Nerchinsk in Siberia, at Murzinka and Miass in the Urals, and Adun Chilon in the Baikal area have produced lovely crystals for many years. However, recent production has waned.
In the United States, Mount Antero in Colorado is well known as a source of beautiful gem aquamarine, and its color compares well with the high quality aquamarine from Africa. Maine, Connecticut, North Carolina (Hiddenite in Alexander County), and California are localities of limited production. India, Pakistan, Afghanistan, Namibia, and Zimbabwe also produce aquamarine. Deposits in Nigeria and Mozambique are more recent discoveries, where high quality aquamarine of dark color has been unearthed. The Mozambique material can exhibit an especially rich blue color, and a well cut aquamarine from this area is a real treasure. The sources in gem-rich Sri Lanka and Burma yield a great number of fine quality gemstone species, and it is quite surprising that so few aquamarines are found there.
Aquamarine hues range from near colorless to the sea-green of oceans and the bluest of skies. Ferrous iron atoms in the channels of the layered ring structure of the tetrahedra account for the blue color. Any ferric iron present causes tints of yellow. A pure blue is now the most desired color, and many aquamarines are heat-treated after cutting to permanently remove the yellow tones and intensify the blue component. This practice is accepted in the trade and proves very difficult to detect. The brownish-green material yields the deeper blue shades after heat-treatment.
During treatment, the expansion of inclusions and any liquids present could fracture or shatter the gems. Therefore, cutting is advisable to remove inclusions before treatment. When kilns with controlled temperatures are available, the greenish aquamarines can be layered in sand and heated. The temperature is raised in increments to the proper level. Careful monitoring of the process is required, as overheating destroys all color.
In Dr. Sinkankas' Gemstone & Mineral Data Book, he describes a primitive method used in Brazil to heat-treat green beryls and obtain the more popular blue color. The stones are securely embedded in bread dough and baked. The loaves are then allowed to cool before being broken for the retrieval of gems and the consumption of fresh bread.
Aquamarine can be confused with blue topaz, blue tourmaline, sapphire, apatite, euclase, zircon, fluorite, and kyanite; however, simple tests can identify it. It is not synthesized for commercial use. Inexpensive synthetic spinel has been used extensively to simulate aquamarine. The color of aquamarine is replicated in paste (glass) imitations. Doublet and triplet imitations have also been manufactured. The delicate to rich colors of the durable aquamarine make this gem an ideal choice for jewelry.
|Composition:||beryllium aluminum silicate Be 3 Al 2 (Si 6 O 18 )+Cr, Fe|
|Crystal System:||hexagonal; per Schumann, trigonal|
|Colors:||near colorless, blue-green, green-blue|
|Phenomena:||chatoyancy and asterism|
|Diaphaneity:||transparent to opaque|
|Habit:||elongated prismatic crystals; often striated and etched|
|Fracture:||conchoidal to uneven|
|Specific Gravity||varies from 2.67 to 2.71|
|Hardness||7.50 to 8.0|
|Toughness:||very good, but can be brittle|
|Refractive Index||o=1.567 to 1.583; e= 1.572 to 1.590|
|Birefringence:||varies from 0.005 to 0.007|
|Optic Character||uniaxial negative|
|Pleochroism||distinct, almost colorless/blue|
|Ultraviolet Fluorescence||inert; iron quenches fluorescence|
|Spectra||disputed; W Schumann on page 64 and J.Arem on pages 52 and 53|
|Color Filter||no information|
|Solubility||insoluble except in fluoric acid|
|Thermal Traits||avoid thermal shock; remove stone during jewelry repairs|
|Inclusions||elongated primary growth tubes that often house liquids containing gas bubbles and microlite crystals; short negative crystals; "snow-stars" formed by microlite crystals surrounded by droplets of liquid on cleavage planes; cleavage cracks; ilminite; biotite; hematite; muscovite; phlogopite; rutile; pyrite|
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