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Spinel Buyer's 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.
 
Color

Spinels of all colors have been found in various different locations worldwide. The market of today sees reds, pinks, oranges and pale blues, and a few other "fancy colors."

While color preferences are always subjective, the ideal spinel colors display intensity and richness without appearing overcast by black or brown overtones.

Colored by metallic trace elements, spinel can be found in the following colors, which are listed in descending order of value:

 

Clarity

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.

Shape & Cut

Due to the octahedral nature of spinel rough, cushion cuts are frequently seen, taking advantage of the squarish cross-section shape of rough spinel crystals to save on valuable carat weight. Trilliants are crafted out of "macles", which are triangular twinned octahedral crystals. All other types of cuts are seen too, from oval to rounds, as well as other shapes such as emerald cuts, pears and trilliants.

Perfect octahedral crystals are sometimes set into jewelry in their original uncut octahedral states. The Burmese refer to these gems as "nat thwe", meaning "spirit polished." Sometimes "nat thwe" spinels will receive a very light polishing.

Stone Sizes

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.

Sources

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).

Carat Weight

Large size, clean spinels are generally not considered rare, but their frequency is far less than tourmaline or quartz based gems. However, as red and blue spinel colors move closely inline with the colors of top quality Mogok ruby and top quality blue sapphire, specimens occur mainly in smaller sizes. This convergence of smaller sizes with top quality colors, renders 7 Carat+ top color quality specimens to be regarded as rare and highly valuable.

As per usual, as the carat weight of a gem increases, so does its per-carat-price. Large spinels are rarer than smaller spinels, meaning carat prices increase rapidly as carat weights increase. Like virtually all other gems, spinel pricing suffers from a "non-linear-scale of increments", and this is especially seen in larger sized specimens of better qualities.

 
Spinel
  Corundum (Ruby & Sapphire)

Composition

Hardness (Mohs)

Specific Gravity

Refractive Index

Crystal System

Colors



Phenomena

Handling


Enhancements


Synthetic available?

MgAl2O4

8

3.63

1.718

Cubic

Near colorless, red, pink, orange, green, blue, violet, purple. No yellow.

Star (4 & 6 rays), cat’s eye

No special care needed

Generally none; occasionally oiling, dying


Yes

Al2O3

9

4.00

1.762–1.770 (0.008) Uniaxial (–)

Hexagonal (trigonal)

All (except an emerald-green)



Star (6 & 12 rays)

No special care needed

Various, including heat, heat + flux healing, surface-diffusion, irradiation, oiling, dying, glass-infilling

Yes

 
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