Spinel is magnesium aluminum oxide, and in its pure state, it is colorless. Chemical impurities give rise to a range of colors, most typically pink or red, purple, green, and blue. The 22.20-carat rosy pink spinel from Sri Lanka and the 36.10-carat red spinel from Burma owe their color to impurities of chromium. Natural spinel crystals, octahedral in shape, are also pictured in the photograph.
There is historical confusion between spinel and corundum (ruby and pink sapphire) because these two gemstones have many similarities, not just their appearance. Both owe their red color to trace impurities of chromium and form when impure limestone is altered by heat and pressure, and they commonly are found side by side. Many of the world's most famous "rubies" are in fact red spinels. Pure spinel is colorless, but impurities give rise to a range of colors, most typically pink or red, but also purple, green and blue. The major sources of spinel gemstones are Burma, Sri Lanka, and Thailand. Other significant occurrences are Pakistan, Afghanistan, and Russia. This pink spinel is from a new find in Tanzania, and it is the only spinel from that locality in the National Gem Collection to date.
This Burmese bracelet has 98 natural spinel crystals set in a double row in yellow gold. The high luster and perfect octahedral spinel crystals, found in the Mogok region of Burma, are called by the Burmese "Anyon nat thwe", meaning spinels that have been cut and polished by the spirits. Historically, there has been much confusion between the ruby and the spinel. It was not until 1783 that spinel was recognized as a mineral distinct from corundum (ruby and sapphire). Ruby is aluminum oxide, while spinel is magnesium aluminum oxide. They both get their reddish color from impurities of chromium. Red spinels are the most popular in jewelry, but in general the gem-buying public is unfamiliar with spinels. Undoubtedly the historical confusion with ruby has led to its reputation as that gemstone's poor relation. Spinel is also confused with synthetic spinel, sometimes used as a simulant for other gems, commonly seen in less expensive jewelry and widely used in high school and college rings.