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Spinel


A gemstone that was confused with ruby and sapphire for over 1000 years.

red and blue spinel
Spinel occurs in a wide variety of colors. The bright reds and deep blues are spectacular specimens. It is easy to understand how early gem traders confused spinel with ruby and sapphire for over 1000 years. Photos by Rob Lavinsky, iRocks.com. Creative commons license.


The Ruby and Sapphire Impostor



Spinel is a gemstone mineral that has been confused with ruby and sapphire for over 1000 years. Several of the most spectacular spinels ever discovered have been mounted in "crown jewels" and other "jewelry of significance" under the assumption that they were rubies or sapphires.

Spinel occurs in the same bright red and blue colors as rubies and sapphires. Spinel forms in the same rock units, under the same geological conditions and is found in the same gravels. It is not surprising that ancient gem traders thought that these colorful spinels were rubies and sapphires.


Why the Confusion?



Two thousand years ago, gemstone traders did not know that spinel and corundum (the mineral of ruby and sapphire) have different chemical compositions and different crystal structures. Instead, gem traders thought that every bright red gemstone was a "ruby" and every deep blue gemstone was a "sapphire". As a result, lots of spinels are now in very important jewelry collections based on their incorrect identification as a ruby.


The Black Prince's Ruby

The most famous example of a spinel being identified as a ruby is a 170 carat bright red spinel named "The Black Prince's Ruby". The first known owner of this beautiful stone was Abu Sa'id, the Moorish Prince of Granada, in the 14th century. The stone passed through several owners and eventually made its way into the Imperial State Crown of the United Kingdom, where it is mounted immediately above the famous Cullinan II diamond [1, photo].


The Timur Ruby

The "Timur Ruby" is a 352.5 carat bright red spinel that is currently in a necklace of The Royal Collection that was made for Queen Victoria in 1853. The stone was found in Afghanistan and is inscribed with the names and dates of its owners back to 1612. It was part of a group of spinels from the Lahore Treasure presented to Queen Victoria by the East India Company in 1849. [2, photo - use the magnify option to see the inscriptions]


Diagnostic Differences (Spinel, Ruby, Sapphire)



Today gemologists understand that there are significant differences between spinel and corundum (the mineral of ruby and sapphire). The diagnostic differences are summarized below. Optical properties can also be used to distinguish spinel from corundum.

Diagnostic Properties
Property
Spinel Corundum
Composition MgAl2O4 Al2O3
Crystal System isometric hexagonal
Typical Crystal Form octahedrons dodecahedrons hexagonal prisms
Hardness 7.5 to 8 9

Gem traders in Myanmar were the first to recognize spinel as being distinctly different from ruby in the late 1500s. [3] In Europe, spinel continued to be misidentified as ruby until the mid-1800s.


What is Spinel?



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Spinel is an oxide mineral with a composition of MgAl2O4. It is very hard (7.5 to 8 on the Mohs Hardness Scale) and is often found in octahedral crystals. It is typically found in three geologic situations: 1) as crystals in limestones and dolomites that have been subjected to contact metamorphism; 2) irregularly-shaped grains in basic igneous rocks; and, 3) as water-worn pebbles in alluvial deposits.

Spinel is very resistant to chemical and physical weathering. It often occurs in marbles which are much less resistant to weathering. Spinel easily weathers out of the marble and is transported by streams. This places spinel in alluvial deposits which are often worked for gemstones. Most of the spinels of a "ruby red color" are produced from alluvial deposits in Sri Lanka, Thailand, Cambodia, Vietnam, Myanmar, and other countries. Other countries where spinel is mined include: Afghanistan, Nepal, Tajikistan, Australia, Madagascar, Nigeria, and Tanzania.


Uses of Spinel



The only significant use of spinel is as a gemstone. It occurs in a variety of colors (colorless, pink, red, orange, blue, purple, brown, black). The colors that imitate ruby and sapphire are the most popular; along with an orange-red color that known as "flame spinel".

Gem-quality red and blue spinels are very rare. They are much less abundant than rubies and sapphires of similar quality and color. Even with equivalent beauty and greater rarity their prices are much lower than ruby and sapphire. This is an example of how rarity has not determined the price. Spinel is not as valuable because it is not as popular. Spinel has not been strongly promoted by the gem and jewelry trade because its supply is limited and unreliable.

Occasionally an exceptional spinel or a jewelry item of historical significance is sold at auction for a very high price. One necklace containing eleven bright red spinels, totaling 1,132 carats and inscribed by Mughal emperors sold for over $5 million. [6, photo]


Synthetic Spinel



The first synthetic spinel was produced in 1847 by Jacques-Joseph Ebelmen, a French Chemist. [7] Commercial production of synthetic spinels was very limited in the 1800s. However, in the 1930s synthetic spinels in a wide variety of colors were produced to imitate popular gemstones such as aquamarine, zircon, tourmaline, emerald, chrysoberyl and ruby. The colors were produced by introducing metals in trace amounts into the stone by the addition of: cobalt oxide (blue), manganese (yellow), chromium oxide (green), and iron (pink). Careful chemical procedures allowed the manufacturers to control the color of the stones. [8]

These colored synthetic spinels were given trade names such as "Tourmaline Green", mounted in inexpensive settings and sold as "birthstone" jewelry. [8] These synthetic stones were the first encounter with spinel for most of the consumers who purchased them.

In addition to its use as a gemstone, synthetic spinel is also used as a refractory. It is used to produce heat-resistant coatings on metal tools and as an additive in making refractory bricks and ceramics.


The Spinel Mineral Group



The name "spinel" is also used for a broad group of minerals with a general chemical composition of AB2O4. In this formula "A" could be filled by: Mg, Fe+2, Zn, Mn+2, Ni, Co, or Cu. "B" could be filled by: Al, Fe+3, Cr, V+3, Ti+4, Ge or Sb. Examples of spinel group minerals include gahnite, magnetite, franklinite, chromite, chrysoberyl and Columbite-Tantalite as shown in the chart in the right column.


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alluvial spinel Vietnam
Several pretty facet-cut spinels. It is easy to see how spinel can be confused with ruby and sapphire or used as an alternative stone. These spinels are about 4 1/2 millimeters in size and weight a little less than 1/2 carat each. The top three red and pink stones weret cut from material mined in Myanmar. Deep red spinel is rarer than ruby but sells at a fraction of the price. The blue stones below them were cut from material mined in Tanzania.


Physical Properties of Spinel

Chemical Classification oxide
Color colorless, pink, red, orange, blue, purple, brown, black
Streak colorless (harder than the streak plate)
Luster vitreous
Diaphaneity transparent to translucent
Cleavage none
Mohs Hardness 7.5 to 8
Specific Gravity 3.5 to 4.1
Diagnostic Properties hardness, octahedral crystals, vitreous luster
Chemical Composition MgAl2O4
Crystal System isometric
Uses the only significant use is as a gemstone




Samarian Spinel
Photo of the Samarian Spinel, the largest known spinel in the world and part of the Iranian Crown Jewels. It weighs approximately 500 carats. It bears an inscription dating to the mid-1600s attributing its ownership to Jehangir, the Mogul Emperor of India. It was taken from India in the early 1700s, during the Afsharid Conquest. [4] Public domain image.


Catherine the Great Crown
The Great Imperial Crown was made for Empress Catherine II the Great's Coronation in 1762. The large red stone at the crest of the crown is the second largest known spinel, weighing 398 carats. It has been titled: "Catherine the Great's Ruby". [5] Creative Commons image by Hugo Gerard Ströhl.


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Spinel Group Minerals
Mineral
Composition
Spinel MgAl2O4
Gahnite ZnAl2O4
Magnetite Fe3O4
Franklinite (Zn,Fe,Mn)(Fe,Mn)2O4
Chromite FeCr2O4
Chrysoberyl BeAl2O4
Columbite-Tantalite (Fe,Mn)Nb2O6--(Fe,Mn)Ta2O6


Spinel Information
[1] The Black Prince's Ruby, featured in an article titled: The Imperial State Crown on The Royal Collection Trust website, accessed, January, 2013.

[2] The 'Timur Ruby' Necklace, an article on The Royal Collection Trust website, accessed, January, 2013.

[3] Spinel: Collector's Favourite, an article on the International Colored Gemstone Association website, accessed, January, 2013.

[4] The Samarian Spinel, article on the Wikipedia.com website, accessed, January, 2013.

[5] The Russian Crown Jewels, article on the "Famous Diamonds" website showing a photo of the Great Imperial Crown of Russia, accessed, January 2013.

[6] An Imperial Mughal Spinel Necklace, Christie's auction in Geneva from May 18, 2011, webpage accessed January, 2013.

[7] Restoring the Luster of a Once-Loved Gem, article by Victoria Gomelsky on The New York Times website, May 12, 2011.

[8] Synthetic Gemstones: Spinel, Gems: Their Sources, Descriptions and Identification, edited by Michael O'Donoghue, Chapter 24, pages 495-502.


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