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Above image: Samarian Spinel, the largest known spinel in the world and part of the Iranian Crown Jewels. It weighs approximately 500 carats.......
Spinel should not be confused with precious stones like rubies and sapphires, though they have more or less similar appearance . For more than 1000 years, there had been some confusion over these closely resembling minerals. Physically and color-wise there may be some similarity, but chemically Spinel is different from others.
Commonly Spinel is a magnesium aluminum group of minerals with a formula MgAl2O4. It comes in different colors such as blue, green, yellow, brown, black and red and has a hardness of 8 on the Mohs Scale of Hardness, and most of the crystal forms are octahedral - eight faced.Their characteristic feature is conchoidal fracture. Spinel, is commonly associated with Peridotite rock in the uppermost Earth's mantle (roughly 20 km to roughly 120 km), possibly to lower depths depending on the chromium content. At significantly shallower depths, above the Moho boundary line.
Above image: Spinel, Octahedral crystal of Burma origin. Mogok Township, Pyin-Oo-Lwin District, Mandalay Division, Burma ......
It occurs in three geologic settings: 1) as crystals in limestone rocks and dolomite that have been subjected to contact metamorphism; 2) irregularly-shaped grains in basic igneous rocks and, 3) as water-worn pebbles (during transit in the rivers) in alluvial deposits. Because of its high hardness, Spinel is very resistant to chemical and physical weathering. However, the ones that occur in marbles which are much less resistant to weathering, separated out of the marble and transported by streams. Hence, they are found in alluvial deposits which are often worked for gemstones. Most of the spinels of "ruby red color" are produced from alluvial deposits in Sri Lanka, Thailand, Cambodia, Vietnam, Myanmar, and other countries.
The Samarian Spinel, a valuable gemstone is part of the Iranian Crown Jewels and is the largest known spinel in the world, weighing 500 carats (100 g). There is also another Spinal that weighed 270-carat (54 g) with a hole plugged by diamond. The smaller of the two Spinel bears a 350-year-old inscription attributing its ownership to Jehangir, a Mogul Emperor of India. Both were part of the loot plundered by Nadir Shah in Delhi during the Mogul period. Though the Mogul empire declined after 230 years, Jahangir's name is very much there on the historical stone's inscription. According to a diary note of the court physician of the Iranian ruler, the Spinel with a hole once adorned the neck of Biblical Golden Calf. It is said that Israelites were said to have made the golden calf while Moses was receiving the Ten Commandments.