Education, Spinel

SPINEL

MINERAL: spinel
CHEMISTRY: MgAl2O4
COLOR: red, orange, pink, purple, blue, black
REFRACTIVE INDEX: 1.718
BIREFRINGENCE: None
SPECIFIC GRAVITY: 3.60
MOHS HARDNESS: 8

Sources

Myanmar, Tanzania, Sri Lanka, Vietnam, Tajikistan.

Birthstones & Anniversaries

Spinel was recently added as an August birthstone, sharing this month with peridot and sardonyx. It has long been mistaken for ruby by emperors and monarchs. Many of the famous “rubies” of history were actually spinels.

Description

The Black Prince’s Ruby. The Timur Ruby. For centuries, spinel, the great imposter, masqueraded as ruby in Europe’s crown jewels
Until recently, spinel was an underappreciated gem with little consumer recognition. Increasing demand for ruby alternatives rekindled appreciation for spinel’s rich red color and history. In ancient times, southeast Asia’s mines yielded exceptional large spinel crystals, which became the treasured property of kings and emperors, often passing through many hands as spoils of war.
Spinel, like garnet and diamond, is singly refractive, with the same physical properties in all crystal directions. It belongs to the cubic crystal system, and its characteristic crystal shape is an octahedron, which looks like two back-to-back pyramids. Well-formed spinel crystals are fairly common in nature.
Spinel can also form flattened crystals that look radically different from octahedral crystals. The flattened shape occurs when the pyramids that form an octahedron rotate against each other during growth. Scientists describe this as a “twinned crystal.” Large gems cut from good-color twinned crystals are typically shallow, and should be judged on their overall beauty rather than on proportions alone.
The spinel used in jewelry is a small part of a group of minerals that share the same crystal structure. Not all of them form transparent crystals suitable for jewelry use, however. Spinel offers a range of hues, from orange to intense “stoplight” red, vibrant pink, and all shades of purple, blue, and violet through bluish green.
Intense reds and pinks are caused by traces of chromium. The higher the chromium content, the stronger the red hue. Orange and purple stones owe their color to a mixture of iron and chromium.
Violet to blue spinel can be colored by trace amounts of iron, and vibrant blues owe their saturated color to trace amounts of cobalt.

ACCESSORIES

History

Spinel is a good candidate for the title of “History’s Most Underappreciated Gem.” Some ancient mines that supplied gems for royal courts from Rome to China produced spinel, but it was usually confused with better-known stones like ruby and sapphire.
In ancient times, the mines of central and southeast Asia yielded exceptionally large spinel crystals. These fine stones became known as Balas rubies, and some of them were the treasured property of kings and emperors, often passing through many hands as spoils of war. As a result, some of the world’s most illustrious “rubies” are actually spinel.
One of the most famous examples is the so-called “Black Prince’s ruby.” This historic crimson-red gem is set in England’s Imperial State Crown and displayed in the Tower of London. Smoothly polished and roughly octagonal in shape, it was probably mined in the mountains of Afghanistan. It first appeared in the historical records of fourteenth- century Spain, and was owned by a succession of Moorish and Spanish Kings before Edward, Prince of Wales—the “Black Prince”— received the stone in 1367 as payment for a battle victory.
Since then, many other English monarchs—including Henry VIII—have cherished the gem. It’s outlasted them all, surviving fires, attempted theft, and World War II bombing raids, to become—with the Koh-i-Noor diamond—one of the centerpieces of England’s Crown Jewels.
Another large spinel in the Crown Jewels, the “Timur ruby,” weighs over 350 carats. It, too, has a checkered history. Several Persian inscriptions carved into the gem testify to its age.
Modern technology hasn’t helped spinel’s confused identity as far as the general public is concerned. This is due largely to the widespread use of synthetic spinel as an imitation for many other gems. Most customers aren’t even aware that there’s a natural version of the stone.

Color

Some spinel colors are more rare and valuable than others. In general, red spinel is the most desirable, followed by fine cobalt-blue spinel, then by vibrant hot pink and vivid orange stones. Violet and bluish purple to purple, or lavender, stones tend to be less attractive, and less in demand than other, rarer colors.
Red spinel ranges from orangy red to purplish red, with pure red to slightly purplish red hues of medium to medium-dark tone considered the finest of all. A top-quality 5-ct. red spinel might sell for around a tenth the price of an equivalent-quality ruby, and pink spinel often sells for less than pink sapphire.
Spinel has been making a name for itself over the last couple of decades, and demand for fine stones well exceeds supply. In fact, spinel is in great demand among gemologists and gem connoisseurs. This has, to some degree, closed the spinel vs. ruby-and-sapphire price gap.
Blue spinel hues range from violet blue through very slightly greenish blue.
Most stones have low saturation, and the blue hues take on a distinctly grayish look. The best and most highly valued blue spinel colors parallel blue sapphire, with intense violet-blue to pure blue colors that are neither too dark nor too light.

Cut

Spinel is cut in a variety of shapes and cutting styles. As with many colored stones, ovals and cushion shapes are very popular.
Because of the scarcity of spinel on the market, most fine-quality rough is cut in non-standard sizes to save weight, instead of in standard industry sizes. The standard-cut, or calibrated, stones that are available tend to be mixed-cut ovals, usually in 6×4-mm and 7×5-mm sizes, suitable for center stones in rings. Commercial qualities are more commonly cut to standard sizes for jewelry.

SPINEL QUALITY FACTORS

The most valued spinel colors are bright red, cobalt blue, and vivid pink and orange. Pale lavender is more affordable
Spinel with no visible inclusions is preferred. The more prominent the inclusions, the less valuable the gem.
Spinel is most often cut in cushion and oval shapes. When properly proportioned it has excellent brilliance.
Fine-color spinel is rare above five carats. Most fine-quality rough is cut to non-standard sizes to save weight.
Spinel is generally highly sought after by gem connoisseurs, and well- formed spinel crystals are in high demand among collectors.
Spinel comes in a variety of colors, but the most commercially important are red and pink. Mauve- or lilac-colored spinels are also attractive, and some blue spinel colors can be intense.

Clarity

Spinel that has no inclusions visible to the eye is more valuable than spinel with inclusions that can be seen. The more visible any inclusions are, the more the value drops.
Spinel can have some very beautiful inclusions that reflect the gem’s usual octahedral crystal growth. Some appear in groups that resemble human fingerprints.

Size and Weight

Larger spinel sizes rise considerably in per-carat price. Availability drops and prices rise sharply for fine red, pink, and blue stones above five carats.

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