The following 7 pages are in this category, out of 7 total. This list may not reflect recent changes (learn more).
1. Tanzanite – Tanzanite is the blue/violet variety of the mineral zoisite belonging to the epidote group. It was discovered by a Tanzanian Jumanne Mhero Ngoma in the Mererani Hills of Manyara Region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. Tanzanite is used as a cheap gemstone, where it can substitute for the far more expensive sapphire after undergoing artificial heat treatment to form a deep blue coloration. Naturally formed tanzanite is rare and is only to the Mererani Hills. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet, Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination, Tanzanite is usually a reddish brown in its rough state, requiring heat treatment to bring out the blue violet of the stone. The mineral was named by Tiffany & Co. after Tanzania, in 2002, the American Gem Trade Association chose Tanzanite as a December birthstone, the first change to their birthstone list since 1912. He assumed that the mineral was olivine but after realizing it wasnt. Shortly thereafter, the stones were shown to John Saul, a Nairobi-based consulting geologist, Saul, with a Ph. D. from M. I. T. Hyman Saul brought the samples across the street to the Gemological Institute of America who correctly identified the new gem as a variety of the mineral zoisite, Tiffanys original campaign advertised that tanzanite could now be found in two places, in Tanzania and at Tiffanys. From 1967, a two million carats of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government in 1971. In 1990, the Tanzanian government split the Tanzanite mines into four sections, Blocks A and C were awarded to large operators, while Blocks B and D were reserved for the local miners. In 2005 the government renewed the lease of Block C mine to Tanzanite One, in June 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India. The ban has been rationalized as an attempt to spur development of local processing facilities, thereby boosting the economy and this ban was phased in over a two-year period, until which time only stones over 0.5 grams were affected. In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram, Tanzanite One Mining Ltd is owned by Richland Resources, but a 2010 law in Tanzania required them to cede 50% ownership of their mining license to the Tanzanian State Mining Company. Production in 2011 amounted to 2.4 million carats, earning them $24 million, the worlds largest rough tanzanite was a 16,839 carat stone mined by TanzaniteOne in 2005. There is no accepted method of grading colored gemstones. TanzaniteOne, a major player in the tanzanite market, through its non-profit subsidiary
2. Epidote – Epidote is a calcium aluminium iron sorosilicate mineral. The faces are often deeply striated and crystals are often twinned, many of the characters of the mineral vary with the amount of iron present for instance, the color, the optical constants, and the specific gravity. The color is green, grey, brown or nearly black and it displays strong pleochroism, the pleochroic colors being usually green, yellow and brown. Clinozoisite is green, white or pale rose-red group species containing very little iron, the name, due to Haüy, is derived from the Greek word epidosis which means addition in allusion to one side of the ideal prism being longer than the other. Epidote is an abundant rock-forming mineral, but one of secondary origin and it occurs in marble and schistose rocks of metamorphic origin. It is also a product of alteration of various minerals composing igneous rocks. A rock composed of quartz and epidote is known as epidosite, the perfectly transparent, dark green crystals from the Knappenwand and from Brazil have occasionally been cut as gemstones. Belonging to the same group with epidote are the species piemontite and allanite. Piemontite occurs as small, reddish-black, monoclinic crystals in the mines at San Marcel, near Ivrea in Piedmont. The purple color of the Egyptian porfido rosso antico is due to the presence of this mineral, allanite and dollaseite- have the same general epidote formula and contain metals of the cerium group. The crystallographic and optical characters are similar to those of epidote, the pleochroism is strong with reddish-, yellowish-, and greenish-brown colors. Although not a mineral, allanite is of fairly wide distribution as a primary accessory constituent of many crystalline rocks, gneiss, granite, syenite, rhyolite, andesite. It was first found in the granite of east Greenland and described by Thomas Allan in 1808, allanite is a mineral readily altered by hydration, becoming optically isotropic and amorphous, for this reason several varieties have been distinguished, and many different names applied. Orthite was the name given by Jöns Berzelius in 1818 to a form found as slender prismatic crystals, sometimes a foot in length, at Finbo. Dollaseite is less common, famous from the Ostanmossa mine in the Norberg district of Sweden, dallasite This article incorporates text from a publication now in the public domain, Chisholm, Hugh, ed. article name needed
3. Allanite – Allanite is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare earth elements. The mineral occurs mainly in metamorphosed clay rich sediments and felsic igneous rocks. It has the general formula A2M3Si3O12 where the A sites can contain large cations such as Ca2+, Sr2+, and rare earth elements and the M sites admit Al3+, Fe3+, Mn3+, Fe2+, or Mg2+ among others. However, an amount of additional elements, including Th, U, Zr, P, Ba, Cr. Allanite contains up to 20% rare earth elements and is a source of them. The inclusion of thorium and other elements in allanite results in some interesting phenomena. Allanite often has a halo of radiation damage in the minerals immediately adjacent. Also highly radioactive grains of allanite often have their structure disrupted or are metamict, the age of allanite grains that have not been destroyed by radiation can be determined using different techniques. Allanite is usually black in color, but can be brown and it is often coated with a yellow-brown alteration product, likely limonite. It crystallizes in the system and forms prismatic crystals. It has a Mohs hardness of 5. 5–6 and a gravity of 3. 5–4.2. It is also pyrognomic, meaning that it becomes incandescent at a low temperature of about 95 °C. It was discovered in 1810 and named for the Scottish mineralogist, the type locality is Aluk Island, Greenland where it was first discovered by Karl Ludwig Giesecke
4. Piemontite – Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca23O. It is a member of the epidote group, red to reddish-brown or red-black in colour, Piemontite has a red streak and a vitreous lustre. The type locality is the Prabornaz Mine, in Saint-Marcel, Aosta Valley and it occurs metamorphic rocks of the greenschist to amphibolite metamorphic facies and in low-temperature hydrothermal veins in altered volcanic rocks. It also occurs in metasomatized deposits of manganese ore, associated minerals include, epidote, tremolite, glaucophane, orthoclase, quartz and calcite
5. Clinozoisite – Clinozoisite is a complex calcium aluminium sorosilicate mineral with formula, Ca2Al3O. It forms a solid solution series with epidote by substitution of iron in the aluminium and is also called aluminium epidote. Clinothulite is a manganese bearing variety with a pinkish hue due to substitution of Mn in the aluminium site and it was originally discovered in 1896 in East Tyrol, Austria, and is so-named because of its resemblance to zoisite and its monoclinic crystal structure. It occurs in rocks which have low to medium grade regional metamorphism. It also occurs in saussurite alteration of plagioclase, introduction to Mineralogy,2000 Oxford University Press ISBN 0-19-510691-1