Spinel
Spinel is the great pretender of the gem world, having been tagged for centuries with the name of every species except the correct one.  Correctly identified in 1783 as a separate mineral from ruby, the misconceptions about Spinel still abound.  Although much confusion has arisen concerning Spinel’s natural pedigree, it is actually a beautiful, durable and valuable natural gemstone in its own right.
 
Spinel can exist in any color; however, it is the red Spinel that is most commonly mistaken for ruby.  The more exposure this gemstone receives, the more apparent it is that there is a considerable color difference between the two.  Red Spinel tends to have a more brownish-red hue and is often referred to as flame Spinel.  When judging Spinel for quality, brightness and purity of color are very important, as the color in Spinel tends to be modified by a gray component due to its chemistry.  Reds, pinks and blues are most affected by this gray color modifier.  Spinel has a dispersion of 0.020, slightly higher than that of sapphire at 0.018.  This dispersion helps contribute to the characteristic bright and lively appearance that the best Spinels display.  Cobalt Spinel possesses a very distinctive intense rich blue color and initially can be mistaken for a fine sapphire, but the Spinel is a brighter, more even blue with none of the color zoning sometimes displayed by their sapphire counterparts.
 
Spinels have traditionally been confused with ruby, and many historic gemstones in royal collections have been tagged with the ruby label only to turn out to be spectacular Spinels upon more informed inspection.  The "Black Princes’" Ruby actually is a red Spinel, an approximately 170 carat tumble polished partially drilled gemstone that currently resides in the British Imperial State Crown at the Tower of London.  Set into the drill hole of this historic red Spinel is a similarly red natural ruby.   
 
The best red Spinels are found in Burma (also known as Myanmar) generally in a pure white calcite host rock.  A new source for incredibly vivid red and pink Spinels has recently been found in Tanzania, in the Morogoro province near Mahenge.  Lovely cobalt blue Spinels also have been found recently at Tunduru in Tanzania.  Equally beautiful cobalt blue Spinels have been mined from Vietnam.  Other sources for Spinel are Sri Lanka, Thailand, Cambodia, Afghanistan, and historically Tajikistan (Badakhshan province) near the Afghanistan border.
 
Spinel Properties
Species:  Spinel
Varieties:  Star, Cats’ Eye, Change of Color
Chemical Composition:  MgO Al2O3
Color:  Colorless (extremely rare), Black, Gray, Yellow, Orange, Red, Pink, Violet, Blue, Green
Hardness:  8
Crystal System:  Cubic
Refractive Index:  1.718
Specific Gravity:  3.58 - 3.61
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