A gemstone that was confused with ruby and sapphire for over 1000 years.
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.
Physical Properties of Spinel
|Color||Colorless, pink, red, orange, blue, purple, brown, black|
|Streak||Colorless (harder than the streak plate)|
|Diaphaneity||Transparent to translucent|
|Mohs Hardness||7.5 to 8|
|Specific Gravity||3.5 to 4.1|
|Diagnostic Properties||Hardness, octahedral crystals, vitreous luster|
|Uses||The only significant use is as a gemstone.|
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. 
Diagnostic Properties: Spinel / Corundum
|Typical Crystal Form||Octahedrons, dodecahedrons||Hexagonal prisms|
|Hardness||7.5 to 8||9|
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 in the chart on this page. Optical properties can also be used to distinguish spinel from corundum.
Gem traders in Myanmar were the first to recognize spinel as being distinctly different from ruby in the late 1500s.  In Europe, spinel continued to be misidentified as ruby until the mid-1800s.
What is Spinel?
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 marble, which is 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 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]
The first synthetic spinel was produced in 1847 by Jacques-Joseph Ebelmen, a French Chemist.  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. 
These colored synthetic spinels were given trade names such as "Tourmaline Green," mounted in inexpensive settings and sold as "birthstone" jewelry.  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 Black Prince's Ruby, featured in an article titled: The Imperial State Crown on The Royal Collection Trust website, last accessed July 2016.
 The 'Timur Ruby' Necklace, an article on The Royal Collection Trust website, last accessed January 2013.
 Spinel: Collector's Favourite, an article on the International Colored Gemstone Association website, last accessed July 2016.
 The Samarian Spinel, article on the Wikipedia.com website, last accessed July 2016.
 The Russian Crown Jewels, article on the "Famous Diamonds" website showing a photo of the Great Imperial Crown of Russia, last accessed July 2016.
 An Imperial Mughal Spinel Necklace, Christie's auction in Geneva from May 18, 2011, webpage last accessed July 2016.
 Restoring the Luster to a Once-Loved Gem, article by Victoria Gomelsky on The New York Times website, May 12, 2011.
 Synthetic Gemstones: Spinel, Gems: Their Sources, Descriptions and Identification, edited by Michael O'Donoghue, Chapter 24, pages 495-502.
The Spinel Mineral Group
The name "spinel" is also used for a broad group of minerals with a general chemical composition of XY2O4. In this formula "X" could be filled by: Mg, Fe+2, Zn, Mn+2, Ni, Co, or Cu. "Y" 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 below.
Spinel Group Minerals
Contributor: Hobart King
|Rock, Mineral and Fossil Collections.|
|Mohs Hardness Scale|
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