Spinel

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Spinel
Chemical composition Mg(Al2O4) Magnesium aluminum oxide
Crystal system Cubic
Habit Octahedral, contact twins
Cleavage Imperfect
Fracture Conchoidal, uneven
Hardness 8
Optic nature Isotropic
Refractive index 1.712 - 1.736
Birefringence None
Dispersion 0.026
Specific gravity 3.58 - 3.61
Lustre Vitreous
Pleochroism None
Spinel from Badak, Afghanistan

Spinel is a mineral species. For many centuries, most gem spinels were misidentified as sapphire or ruby because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.
The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).

Sapphire history

Cobalt Blue Spinel, 3.96 ct
Photo by Jeff Scovil
Courtesy of R.W. Wise Goldsmiths