shallower depths, above the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite while garnet is the stable phase deeper in the mantle below the spinel stability region. Spinel, , is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites. Geographical occurrence Spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of the Badakshan Province in modern-day Afghanistan and Tajikistan; and of Mogok in Myanmar. Over the last decades gem quality spinels are found in the marbles of Lục Yên District (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar). Since 2000, in several locations around the world, spinels have been discovered with unusual vivid pink or blue colors. Such "glowing" spinels are known from Mogok (Myanmar), Mahenge plateau (Tanzania), Lục Yên District (Vietnam) and some more localities. In 2018 bright blue spinels have been reported also in the southern part of Baffin Island (Canada). The pure blue coloration of spinel is caused by small additions of cobalt. Synthetic spinel Synthetic spinel can be produced by similar means to synthetic corundum, including the Verneuil method and the flux method pioneered by Edmond Frémy. It is widely used as an inexpensive cut gem in birthstone jewelry for the month of August. Light blue synthetic spinel is a good imitation of aquamarine beryl, and green synthetic spinel is used as an emerald or tourmaline simulant. By 2015, transparent spinel was being made in sheets and other shapes through sintering. Synthetic spinel, which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use. See also Aluminium oxynitride Ceylonite The Three Brothers, a lost 14th-century crown jewel with three red spinels in a triangular arrangement References Bibliography Deer, Howie and Zussman (1966). An Introduction to the Rock-Forming Minerals, Longman, pp. 424–433, . Shumann, Walter (2006). Gemstones of the World 3rd edition, Sterling, pp. 116–117. External links Spinel structure at the University of Wisconsin - Green Bay Spinel structure at the Institut for materials science of the University of Kiel Value of Spinel Aluminium minerals Cubic minerals Minerals in space group 227 Magnesium minerals Luminescent minerals Transparent materials