The mineral forms in metamorphosed ancient sea beds, where aluminum-rich thermal waters ran through the rock. Metamorphic rocks are also an important source of corundum. In particular, the melt has to be rich in aluminum (Al) but devoid of silicon (Si), one of the most common elements in the Earth’s crust. The slower the rock cools, the larger the crystal can grow.Ĭorundum won’t develop in just any igneous rock. Rubies are red due to the presence of chromium, and sapphires can be a variety of colors. While not visible in the colorless variety of corundum, it is a concern for sapphire and ruby cutters since it makes the gem show different colors in different directions. How Does Corundum Form?Ĭorundum (chemical formula Al 2O 3) crystalizes in slow-cooling igneous or metamorphic rocks. The two most common corundum gem varieties are ruby and sapphire. Despite the title of ruby (a variety of corundum) the ruby is actually a piece of spinel. This is the chapter on ruby and sapphire inclusions from Richard Hughes' 1997 book, Ruby & Sapphire. Non-gem quality corundum, like that used commercially as an abrasive, is simply called corundum. Ruby & Sapphire (1997) Inclusions in Corundum 1 January 1997 By Richard Hughes Inclusions are among the most important features for identifying ruby and sapphire. All other colors of gem-quality corundum are considered sapphires. The biggest difference between ruby gemstones and other varieties of corundum is they usually do not grow as large. The gem varieties of Corundum are Ruby and Sapphire. So, if ruby and sapphire are the same mineral, why do we distinguish them? They share the same gemological properties with only one distinction: rubies are gem-quality red corundum. Find many great new & used options and get the best deals for 1.66ct GIA CERTIFIED NATURAL CORUNDUM GEMSTONE SAPPHIRE-RED at the best online prices at eBay. Not surprisingly, when combined with fantastic colors, rubies and sapphires are some of the most highly desired gemstones. Did you know that ruby and sapphire are actually the same mineral, corundum? Among natural minerals, only diamond has a greater hardness.
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