Spinel Buying Guide
SPINEL BUYING GUIDE
Throughout history, spinel has been confused with ruby. In part, this is because spinel is often found in the same deposits. Gem spinel is a magnesium aluminum oxide, while ruby (corundum) is an aluminum oxide. In deposits where both ruby and red spinel are found, spinel is typically more common than ruby. This is because, when both magnesium and aluminum are present, spinel grows. Only after the magnesium is exhausted, does ruby get a chance to crystallize. In addition, both ruby and red spinel owe their color to the same Cr+3 ion.
It is most likely that the famous Mogul spinels as described in Dr. Ball’s article above originated from mines long-since abandoned along the Amu Darya (Oxus) river that separates present-day Tajikistan from Afghanistan. The locality is just inside Tajikistan, about 47 km. south of Khorog, on the edge of the Pyandzeh river valley.
Fine examples of these “Badakhshan” spinels can be found in some of the most famous gem collections in the world. Perhaps the largest single grouping is in the Crown Jewels of Iran in Teheran. Others can be found in the British Crown Jewels in the Tower of London and the Kremlin in Moscow.
For red spinel, the finest colors tend to be similar to ruby, i.e. a rich, intense red similar to that of a red traffic signal. However, spinel tends to be a bit more of a brick red than ruby (which is slightly more purplish). Like all gems, the most-highly sought are those whose color is intense, while being not too light or dark. Prices decline as the color diminishes into either light pinks or “garnety” reds.
Orange spinels (also known as ‘flame’ spinels) can also be magnificent. Look for stones that are a rich orange without too many blackish overtones.
Sri Lanka rarely produces blue spinels colored by cobalt. Such “cobalt” blue spinels are highly sought after by collectors. The best pieces are an intense blue unique in the world of gems.
In addition to red, pink, orange and blue, spinel is found in a myriad of other colors. These are generally termed “fancy” spinels, and occur most often in shades of mauve, violet, purple and blue-green.
In terms of clarity, spinel is often cleaner than ruby. However, the very finest reds are so rare that some clarity defect is almost always present (usually fractures). Included crystals are quite common in spinel. Many stones display natural iron-oxide stains in their fractures.
Due to the octahedral nature of spinel rough, the most common shape seen is the cushion. Rounds are also seen, as are other shapes, such as the emerald cut.
Prices. Red spinel has always resided in the shadow of ruby, with the result that prices are just a fraction of what the equivalent ruby would cost. Prices of intense cobalt-blue spinels can rival, or even exceed, those for the finest reds. Because synthetic spinels are often used for imitation birthstone rings, many people think “synthetic” when they hear the name “spinel.” This is unfortunate, for a fine gem spinel is one of nature’s most beautiful and rare treasures.
In fact, the main thing holding back greater recognition for spinel is rarity.
While faceted spinels of 100 carats or greater are known, top red or blue stones in sizes above five carats are rare. Fancy spinels in sizes up to 20 carats or more are generally available.
Spinel typically occurs as octahedra or twinned octahedra (macles), either well-formed or flattened. Specimens from Burma’s Mogok Stone Tract are often so perfect that they are mounted in jewelry as is. Such stones are termed nat thwe (‘spirit polished’).
The name spinel is believed to derive from the Latin word, spina, meaning “thorn,” perhaps in allusion to the pointed shape typical of spinel octahedra. In the past, spinel has been referred to as “balas ruby”, a reference to the mines of Badakhshan.
Today, fine spinels come from a handful of sources. The best red, pink and orange spinels originate from the rich gem gravels of Burma’s Mogok Stone Tract. The best blue and violet spinels are found in Sri Lanka (Ceylon). Gem spinel is also found in Vietnam, the Pamir mountains of Tajikistan, Tanzania and Madagascar. Black spinel is mined in Thailand (at Bo Ploi, Kanchanaburi).
Synthetic spinel has been produced by the Verneuil process since about 1909. It is made in a variety of colors, some of which are not found in nature. Spinel has also been grown in Russia by the flux process in red and cobalt-blue colors.
Properties of spinel and corundum
(Ruby & Sapphire)
|Refractive Index||1.718||1.762–1.770 (0.008) Uniaxial (–)|
|Crystal System||Cubic||Hexagonal (trigonal)|
|Colors||Near colorless, red, pink, orange, green, blue, violet, purple. No yellow.||All (except an emerald-green)|
|Phenomena||Star (4 & 6 rays), cat’s eye||Star (6 & 12 rays)|
|Handling||No special care needed||No special care needed|
|Enhancements||Generally none; occasionally oiling, dying||Various, including heat, heat + flux healing, surface-diffusion, irradiation, oiling, dying, glass-infilling|
From this table, one can see how similar spinel and ruby are. Thus the confusion.
For historical information on Badakhshan spinels, as well as technical information about spinel, see also:
The rubies and spinels of Afghanistan – A brief history by Richard W. Hughes
Spinel compounds: Background and Historical Perspective by Kurt Sickafus and Richard W. Hughes