Spinel is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Cubicly structured gems are made of magnesium aluminum oxide, their full chemical compound being MgAl2O4.
Spinel is an extensive group of minerals in which magnesium may be replaced with iron, zinc, and above all, manganese. The hardness and fine colors of gem quality spinel, known as Noble Spinel, set it apart as a gem material from other types of spinel.
It normally occurs as distinct octahedral crystals, as clusters also of octahedral habit, or as characteristic twins. The crystals are often isolated, sometimes in aggregates, 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 that of the corundum group, with its sapphires, rubies, etc.
Spinel is formed by regional metamorphism or contact metamorphism in limestones, and also schists. It is also found in pegmatitic, pneumatolytic environments, together with corundum, and even in alluvial deposits where rolled pebbles of spinel may be found.
It mainly occurs in Afghanistan, India, Burma, or the gem gravels of Sri Lanka, Madagascar, and Brazil.
The varieties of spinel are:
Red spinel -
This spinel may be an intense, bright red like ruby, but more often may appear softer than red, as in pink-red, brick red, or almost orange. It can also have a violet tinge. Such stones were formerly referred to as Balas ruby, after the Badakshan (Balascia) region of Afghanistan where they were found.
The red color of spinel becomes more intense in bright light, but much less than ruby. It is singly refractive, so is not pleochroic. This feature is in contrast to ruby but in common with garnet, although the latter is nearly always a rather dull color, which is not heightened even by strong light.
Red and pink spinels come from the Mogok region of Burma and Afghanistan, and may also come from Sri Lanka and Thailand, where they are found together with corundum. When rubies and spinels are found side by side, it means that the aluminum oxide in the surrounding rocks has either formed into the corundum mineral, or combined with magnesium to form spinel.
Blue spinel -
The blue variety of noble spinel was in the past much less widely known and appreciated than the red variety.
The finest specimens, though rare, have a bright blue color comparable to that of some sapphires and are very attractive, lustrous, and transparent. They are almost as pleasing are equally lustrous light violet-blue stones, and resemble some sapphires of the same color.
Fine blue spinels are hard to distinguish at first sight from sapphires of a similar color. A difference in pleochroism is not always a sure distinction, as there spinels that have clearer pleochroism. But there is a marked variation in density, which is easy to establish. Cloudy, deep-blue spinels, of course, are easy to distinguish.
Blue spinel is mainly found in the gem gravels of Sri Lanka, and rarely, in Burma. Recently it has also been found in Tanzania, Madagascar, Kenya, and Vietnam.
Ceylonite or pleonaste - The dark green to black opaque spinel
Piconite - the brown variety
Rubicelle - the yellow variety
Spinel may derive its name from the Latin "spina" or "thorn," referring to the triangular shape of the crystal faces, or from the Greek root "spinter," meaning "spark". Like ruby and garnet, it has also been called "carbuncle" from the Latin "carbunculus" or "small coal."
A bit of unintentional confusion has been created by the accustomed name of "ruby spinel." It turns out that some of the largest and most famous "rubies" in the world, such as the 361carat "Timur ruby" and the "Black Prince's Ruby" in the English crown jewels are actually spinels, and not corundums. Both are uncut and only polished. The drop-shaped spinels in Bavaria's Wittelsbach crown of 1830 were also originally thought to be rubies. Nonetheless, spinel deserves to be valued as a gem in its own right.
In earlier times, perfectly octahedral crystals were used in jewelry as they were, uncut, like diamonds. Stones found as worn pebbles or irregular pieces were rounded and then polished, like some large spinels now seen in museums. The two largest spinels (formed as roundish octahedrons) weigh 520 carats each and are in the British Geological Museum in London.
Spinel was recognized as an individual mineral only 150 years ago. Before then it was classified as a ruby, because it also occurs with it.
Spinel has good luster and transparency. It is generally given a mixed, oval, or round cut; alternatively, a square or rectangular, step or trap cut.
As far as secondary gems go, brilliant red spinels comparable in color to rubies are quite high in value, though they are only one-tenth the price of rubies. Pale pink or violet spinels, unless exceptionally fine, or large specimens, are of much lower value.
Synthetic spinel in many colors has been widely produced through flame fusion, but even with this method, the red variety can be difficult to obtain. Thus the only examples found are extremely rare and very small – a maximum of one carat or little more.
Blue spinels have a deep, dull color descending to sooty gray, with a violet tinge, and their luster and transparency is often a bit clouded up. So they are often cut shallow, to try to lighten the color, but this only helps to moderately improve it. Like sapphires, they are normally given a mixed, oval cut. Blue spinels that are attractively colored, which are rare and not very large, are worth much the same as the red spinels. whereas the fairly common, cloudy stones are very modestly priced.
Dark-blue synthetic spinel was one of the first synthetic gems to be manufactured at the beginning of the twentieth century, not to imitate natural spinel, but the much more precious sapphire. Synthetic spinel was colored by adding cobalt (in natural sapphire, the coloring agent is iron). As a result, if one of these altered sapphire-intended spinels is exposed to strong tungsten light, it emits rays of red fluorescence not seen in any other stone, and this can be a valuable aid to distinction. An expert can easily recognize spinel because of the absence of the double refraction in it.