Beryl rough gemstones

BERYL
Three varieties of beryl: Morganite, Aquamarine, and HeliodorThe mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare. Beryl exhibits conchoidal fracture, has a hardness of 7.5–8, a specific gravity of 2.63–2.80. It has a vitreous luster and can be transparent or translucent. Its cleavage is poor basal and its habit is dihexagonal bipyramidal. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, red, and white. The name comes from the Greek beryllos for the precious blue-green color of sea water.

Varieties Varieties of beryl have been considered gemstones since prehistoric times. Recognized for its beauty, in the Bible, in Ezekiel 1:16, the wheels of God's throne are described as having the appearance of "gleaming beryl". Green beryl is called emerald, red beryl is bixbite or red emerald or scarlet emerald, blue beryl is aquamarine, pink beryl is morganite, white beryl is goshenite, and a clear bright yellow beryl is called golden beryl. Other shades such as yellow-green for heliodor and honey yellow are common. Red beryl is extremely rare and is not used in jewelry as the crystals it forms are very small. Blue beryl (aquamarine) when exposed to sunlight will not fade in color. Maxixe type beryl is a deep blue stone that fades to white when exposed to sunlight or is subjected to heat treatment, color returns with irradiation

Deposits Beryl is found most commonly in granitic pegmatites, but also occurs in mica schists in the Ural Mountains and is often associated with tin and tungsten orebodies. Beryl is found in certain European countries such as Austria, Germany, and Ireland. It also occurs in Madagascar (especially morganite).

The most famous source of emeralds in the world is at Muzo and Chivor, Boyacá, Colombia, where they make a unique appearance in limestone. Emeralds are also found in the Transvaal, South Africa, Minas Gerais, Brazil, and near Mursinka in Urals. In the United States emeralds are found in North Carolina. New England's pegmatites have produced some of the largest beryls found, including one massive crystal with dimensions 5.5 m by 1.2 m (18 ft by 4 ft) with a mass of around 18 metric tons. It is New Hampshire's State Mineral. Other beryl locations include South Dakota, Colorado, Utah, Idaho, and California.

As of 1999, the largest known crystal of any mineral in the world is a crystal of beryl from Madagascar, 18 metres long and 3.5 metres in diameter.

Applications Massive beryl is a primary ore of the metal beryllium.

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Spinel Gemstones from Tanzania

The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. The general formula is as (X)(Y)2O4, with X representing cations occupying tetrahedral sites and Y cations occupying octahedral sites. Divalent, trivalent, and quadrivalent cations can occupy the X and Y sites, and they include magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon.

The oxygen anions are arranged in a cubic close-packed structure. In the normal spinel structure, X cations occupy the tetrahedral sites, and Y the octahedral sites. For inverse spinels, half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy the octahedral sites.

Important members of the spinel group include:

Spinel – MgAl2O4, after which this class of minerals is named Gahnite - ZnAl2O4 Franklinite - (Fe,Mn,Zn)(Fe,Mn)2O4 Chromite - (Fe·Mg)Cr2O4 Magnetite - Fe3O4 Hercynite - FeAl2O4 Ulvöspinel - TiFe2O4 Jacobsite - MnFe2O4 Trevorite - NiFe2O4 Ringwoodite - SiMg2O4, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand.

Spinel usually occurs in isometric crystals, octahedrons, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull lustre. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Another famous spinel is the Black Prince's Ruby in the British Crown Jewels.

The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Kokcha river, one of the principal tributaries of the Oxus river. Yellow spinel is called rubicelle and violet-colored manganese-bearing spinel is called almandine.

Spinel is found as a metamorphic mineral, and also as a primary mineral in basic rocks, because in such magmas the absence of alkalis prevents the formation of feldspars and any aluminium oxide present will form corundum or combine with magnesia to form spinel. This is why spinel and ruby are often found together.

Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite in the uppermost Earth's mantle, between the Mohorovicic discontinuity (the Moho) and a depth of 70 kilometers or so; below that depth, the spinel (if present) becomes increasingly rich in chromium, as with increasing depth, pyrope-rich garnet becomes the more stable aluminous mineral in peridotite. At depths significantly shallower than the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite.

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