Thursday, 14 February 2013

Introduction to Spinel Gemstone

Crystal of noble spinelMagnesium aluminum oxide. The spinels are an extinctive group of minerals, in which Mg may be replaced by Fe, Zn and above all, Mn; and Al may be replaced by Fe of If the hardness and fine colors of gem quality spine! (Also known as noble spinet) distinguish it as a gem material other types of spinel.

Crystal system Cubic

Appearance


It normally occurs as distinct octahedra, as clusters also of octahedral habit, or as characteristic twins. The crystals are often isolated, sometimes aggregated fairly complete, lustrous, and with a color varying from red, to pink, violet red, pale lilac, violet blue, blue, or black. The possible color range is almost as extensive as t' corundum. It can be found as rolled pebbles in alluvium

Physical properties


Spinel has a hardness of 8, singly refractive, with an index of 1.71 or a little more the density is 3.58-3.61 9/cm3.

Genesis


Red spinelSpinet is formed by regional metamorphism contact metamorphism in limestones, and also schist. it also found in pegmatitic, pneumatolytic environment together with corundum, and in alluvial deposits.

Occurrence


It mainly occurs in Afghanistan, India, Burma, or the gem gravels of Sri Lanka, Madagascar and Brazil.

Red spinel


The name spinet may come from the Latin spina or "thorn,- referring to the triangular shape of the crystal, faces, or from the Greek root spinter, meaning spark presumably referring just to the red variety. Like ruby and garnet, it has also been called "carbuncle," from the Latin carbunculus, or "small coal." However, recognition the various types of carbuncle differed in hardness led to their being valued accordingly. Undesirable confusion has been created by the alternative name of ruby spinet; are fact, some of the largest and most famous "rubies' in the world, such as the "Black Prince's Ruby- in the English crown jewels, are really spinets, not corundums.

Appearance


Violet spinelSpinet may be an intense, bright reds ruby, but more often tends to be brick red, almost orate It can have a violet tinge, and such stones were forma referred to as Batas ruby, after the Badakshan region of Afghanistan where they were found. The cold often soft: pink, rather than red. Spinet has good ho and transparency. It is generally given a mixed, oval (round cut; alternatively, a square or rectangular, steps or trap cut.

In earlier times, perfectly octahedral crystals were used in jewelry as they were, uncut, likes diamonds. Stones to as worn pebbles or irregular pieces were summa rounded and then polished, like some large spinels seen in museums.

Distinctive features


Red spinet is fairly similar to rut the red color fluoresces slightly in bright light, but much less than ruby. Being singly refractive, it is not pleochroic. This feature is in contrast to ruby but in common with net, although the latter is nearly always a rather dull color which is not heightened even by strong light. It can be to distinguish spinet from either of these gemstones which case the physical properties will need to be measured. However, spinet may contain several parallel of minute octahedral crystals of hercynite (iron which are highly distinctive and sometimes recognizable with an ordinary lens. Soft pink varieties can be hard distinguish from some tourmalines, although the display varying degrees of birefringence, and have different physical properties.

Occurrence


Pink spinelRed and pink spinels come from !held region of Burma and Afghanistan. Spinets may also from Sri Lanka and Thailand, where they are found together with corundum.

Value


As secondary gems go, brilliant red spinels parable in color to rubies are quite valuable, though are only one-tenth or so the price of rubies themselves Pale pink or violet spinels, except for particularly fine large specimens, are of much lower value.

Simulants and synthetics


Synthetic spinel in colors has been widely produced by the flame method, but owing to difficulties in obtaining the red 'variety, the only examples found are extremely rare and small size a maximum of one carat or little more, are distinguished from the natural variety by the typical internal structure and inclusions of gems produced by the Verneuil method. Recently, synthetic red spinel has been produced by the more costly flux melt process. but has not proved economical.

Blue spine!


The blue variety of noble spinet was in the past much less widely known and appreciated than the red variety.

Appearance


Dark blue spinelThe finest specimens, though rare have bright blue color comparable to that of some sapphires and are very attractive, lustrous, and transparent. Almost pleasing are equally lustrous light violet-blue stones resembling some sapphires of the same color. Most spinels, however, is a deep, dull color tending to sooty gray, albeit with a violet tinge; and their luster and transparency are marred by a touch of cloudiness. These are often cut quite shallow, in an attempt to lighten the cot usually with modest results. Like sapphires, they arena malty given a mixed, oval cut.

Distinctive features


Fine blue spinets are hard to distinguish at first sight from sapphires of a similar color. T., latter may display clear pleochroism, but this is not ales. The case, particularly with medium to light blue specimens Hence lack of pleochroism is not a sure distinction. 8. there is a marked variation in density, which is easy to establish, and the refractive indices are also different Cloudy, deep-blue spinets, on the other hand, are quite distinctive.

Occurrence


Blue spinet is mainly found in the gem gravels of Sri Lanka and, rarely, in Burma.

Value


Sky blue spinelAttractively colored blue stones, which ate, and not very large, are worth much the same as the red variety, whereas the fairly common, cloudy, less an stones are very modestly priced.

Simulants and synthetics


Like synthetic ruby, clam synthetic spinet was one of the first synthetic gems manufactured, at the beginning of the twentieth century Synthetic stones were not intended to imitate spinet, but the much more precious sapphire, alter unsuccessful attempts to synthesize the latter.  

Synthetic spinet was colored by adding cobalt (inn sapphire, the coloring agent is iron). As a result, eon if one of these stones is exposed to strong, tungsten light, it e strange gleams of red fluorescence of a type not seen in any other stone, and this can be a valuable aid to r lion. Light-blue synthetic spinet is also produced in considerable quantities, to simulate another gem: aquamarine.

Writer – Curzio Cipriani and Alessandro Borelli

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