December Birthstone

The three birthstones associated with December are Tanzanite, Zircon, and Turquoise.

Tanzanite

TANZANITE

Discovered in the late 1960s in Tanzania, and found exclusively in this tiny area of the world, tanzanite exhibits a rich violet-blue color for which the gemstone is treasured; often it is heat-treated to achieve this color.  Colors range from blue to purple, and tanzanites that are medium dark in tone, vivid in saturation, and slightly violet blue command premium prices.  As tanzanite can be less expensive than sapphire, it often was purchased as an alternative.  However, it has increased in popularity and now is valued more for its own beauty and brilliance than as a sapphire substitute.

Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) discovered in the Mererani Hills of Manyara Region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. It is used as a gemstone. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation. 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 in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone. Tanzanite is a rare gem. It is found only in the foothills of Mount Kilimanjaro. The mineral was named by Tiffany & Co. after Tanzania, the country in which it was discovered. Because it is relatively soft, tanzanite is most commonly set in necklaces and earrings.

Factors affecting value: grading

There is no universally accepted method of grading colored gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its non-profit subsidiary, The Tanzanite Foundation, has introduced its own color-grading system. The new system's color-grading scales divide tanzanite colors into a range of hues, between blue violet and violet blue.

The normal primary and secondary hues in tanzanite are blue and purple, not violet. Purple is a modified spectral hue that lies halfway between red and blue. Tanzanite is a trichroic gemstone, meaning that light that enters the stone is divided into three sections, each containing a portion of the visible spectrum. After heating, tanzanite becomes dichroic. The dichroic colors are purple and blue. The hue range of tanzanite is blue-purple to purple-blue.

Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision). The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-flawless. Gems with eye-visible inclusions will be traded at deep discounts.

Trivia

Tanzanite is the unofficial region gem of Manyara Region, where it was discovered and is still mined.

Zircon

ZIRCON

Derived from the Arabic words zar and gun, meaning gold and color, zircon is found in a wide range of colors such as: blue, yellow, orange, brown, green, colorless, and red (the most prized color). For many years colorless zircon was used to imitate diamonds.  Folk wisdom grants zircon the power to relieve pain, whet the appetite, protect travelers from disease and injury, to ensure a warm welcome, and to prevent nightmares guaranteeing a deep, tranquil sleep.  Major sources of zircon are the Chanthaburi area of Thailand, the Palin area of Cambodia, and the southern part of Vietnam.

The name derives from the Persian zargun (زرگون), meaning golden-colored. This word is corrupted into "jargoon", a term applied to light-colored zircons. The English word "zircon" is derived from "Zirkon," which is the German adaptation of this word. Red zircon is called "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.

Properties

Zircon is ubiquitous in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). Large zircon crystals are rare. Their average size in granite rocks is about 0.1–0.3 mm, but they can also grow to sizes of several centimeters, especially in pegmatites.

Because of their uranium and thorium content, some zircons undergo metamictization. Connected to internal radiation damage, these processes partially disrupt the crystal structure and partly explain the highly variable properties of zircon. As zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, and the color changes.
Zircon occurs in many colors, including red, pink, brown, yellow, hazel, or black. It can also be colorless. The color of zircons can sometimes be changed by heat treatment. Depending on the amount of heat applied, colorless, blue, or golden-yellow zircons can be made. In geological settings, the development of pink, red, and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers. Color in this red or pink series is annealed in geological conditions above the temperature about 350 °C.

Turquoise

TURQUOISE

The name turquoise, from the French expression Pierre tourques or Turkish stone, originated in the thirteenth century and describes one of the oldest known gemstones. Turquoise varies in color from greenish blue, through robin's egg-blue, to sky blue shades and its transparency ranges from translucent  to opaque. Turquoise is plentiful and is available in a wide range of sizes. It is most often used for beads, cabochons, carvings, and inlays. Although its popularity fluctuates in fashion, it is a perennial favorite in the American Southwest.

Properties of turquoise

Even the finest of turquoise is fracturable, reaching a maximum hardness of just under 6, or slightly more than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals and all of its properties are highly variable. Its crystal system is proven to be triclinic via X-ray diffraction testing. With lower hardness comes lower specific gravity (2.60–2.90) and greater porosity: These properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.

The refractive index (as measured by sodium light, 589.3 nm) of turquoise is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemmological refractometer, owing to the almost invariably polycrystalline nature of turquoise. A reading of 1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals. An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nanometres and a weak band at 460 nanometres (this is best seen with strong reflected light). Under longwave ultraviolet light, turquoise may occasionally fluoresce green, yellow or bright blue; it is inert under shortwave ultraviolet and X-rays.

Turquoise is insoluble in all but heated hydrochloric acid. Its streak is a pale bluish white and its fracture is conchoidal, leaving a waxy lustre. Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.

Formation

As a secondary mineral, turquoise apparently forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of pre-existing minerals. For example, the copper may come from primary copper sulfides such as chalcopyrite or from the secondary carbonates malachite or azurite; the aluminium may derive from feldspar; and the phosphorus from apatite. Climate factors appear to play an important role as turquoise is typically found in arid regions, filling or encrusting cavities and fractures in typically highly altered volcanic rocks, often with associated limonite and other iron oxides. In the American southwest turquoise is almost invariably associated with the weathering products of copper sulfide deposits in or around potassium feldspar bearing porphyritic intrusives. In some occurrences alunite, potassium aluminium sulfate, is a prominent secondary mineral. Typically turquoise mineralization is restricted to a relatively shallow depth of less than 20 metres (66 ft), although it does occur along deeper fracture zones where secondary solutions have greater penetration or the depth to the water table is greater.

Although the features of turquoise occurrences are consistent with a secondary or supergene origin, some sources refer to a hypogene origin. The hypogene hypothesis holds that the aqueous solutions originate at significant depth, from hydrothermal processes. Initially at high temperature, these solutions rise upward to surface layers, interacting with, and leaching essential elements from pre-existing minerals in the process. As the solutions cool, turquoise precipitates, lining cavities and fractures within the surrounding rock. This hypogene process is applicable to the original copper sulfide deposition; however, it is difficult to account for the many features of turquoise occurrences by a hypogene process. That said, there are reports of two phase fluid inclusions within turquoise grains that give elevated homogenization temperatures of 90 to 190 °C that require explanation.

Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are exceedingly rare. Typically the form is vein or fracture filling, nodular, or botryoidal in habit. Stalactite forms have been reported. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Odontolite is fossil bone or ivory that has been traditionally thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Intergrowth with other secondary copper minerals such as chrysocolla is also common.


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