Spinel is the magnesium/aluminum member of the larger Spinel Group of minerals. It has the formula MgAl2O4. The Spinel Group minerals are metal oxides. Some of the better-known group members are Magnetite, Chromite, Chrysoberyl, Franklinite, and Gahnite.
Spinel occurs in almost every color including red, pink, purple, blue, green, aqua, orange, yellow, brown, gray, and black. Bright red Spinel, known as Ruby Spinel, is the most valuable variety of this mineral. In the past, there was no distinction between true Ruby and Ruby Spinel, as they look very similar and are found together in the same localities. Some of the world’s most famous Rubies and Sapphires are actually Spinel. This mineral occurs in the same bright red and blue colors as rubies and sapphires. It forms in the same rock units, under the same geological conditions, and is found in the same gravels. The most famous example of this gem 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. Another example is from the Great Imperial Crown which 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. Also, 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.
This mineral was recently (2016) named a new birthstone for the month of August by the American Gem Trade Association and the Jewelers of America. Before then, Peridot served as the August birthstone. Now both minerals will share the designation.
Occurences of Spinel
This mineral occurs in octahedral crystals, often in perfectly shaped octahedrons. Crystals often have parallel layer growths or heavy etchings. Crystals can also be distorted as well as have complex faces. Crystal clusters and twinned octahedrons are very common, especially spinel twinning, which this mineral is famous for. An interesting habit occasionally observed in this specimen is the macle, which is a triangular form of two flattened crystal twins. It also occurs grainy and as rounded, water worn pebbles.
It is a member of the Isometric Crystal System, Hextetrahedral Class. Hardness is 7.5-8.0 on the Moh’s Scale. It is found in metamorphic marbles and hornfels, as well as in plutonic rocks such as gabbro. Also, in alluvial placer deposits. Red, pink, and violet Spinel often fluoresces red in long-wave or neon-yellow in shortwave ultraviolet light.
Most gem grades of this mineral comes from Mogok, Myanmar. Another important producer is Sri Lanka, especially at Ratnapura. The original gems came from Afghanistan, at Badakhshan; newer Afghan localities have recently been exploited in Jegdalek, in Sarobi. A very significant recent producer of Spinel is Luc Yen, in Yenbai Province, Vietnam. The Hunza Valley, Gilgit, Pakistan produces a nice blue Spinel. A famous occurrence of black Spinel crystals and clusters is the Aldan Shield, Yakutia, Siberia, Russia; and transparent purple crystals have come from Kukh-i-Lal, Pamir Mts, Tajikistan.
Madagascar also has several notable localities in Tulear Province, especially at Ambatomainty, where large clusters of black crystals have occurred. Tanzania also has noted occurrences in Ipanko and Morogoro. A European location of note is the Fassa Valley (Val D’Fassa), Trento Province, Italy. Spinel has been found in Canada, in Ross Township, Renfrew County, Ontario; and in the Parker mine, Notre-Dame-du-Laus, Quebec.
In the United States, the finest crystals came from the Franklin Marble region which stretches from Sussex County, New Jersey to Orange County, New York. Classic Spinel localities in that region in New Jersey include the Limecrest Quarry, Sparta; the Sterling Hill Mine, Ogdensburg; and Franklin, Sussex County. In New York, they include Amity, Edenville, and Warwick, Orange County. Amity is perhaps the finest U.S. occurrence, where enormous black octahedrons were found in the 18th century. Enormous Spinel crystals were also found in a lost locality in Monroe, Orange County, New York. Spinel was also found at Bolton, Worcester County, Massachusetts, and opaque blue crystal clusters occur near Helena, Lewis and Clark County, Montana.
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Article by Bill Jones, Sidewinder Minerals