However, the Cr 3+ ions are larger and have electron orbitals in different directions than aluminium. When a chromium atom replaces an occasional aluminium atom, it too loses 3 electrons to become a chromium 3+ ion to maintain the charge balance of the Al 2O 3 crystal. The substitution density of Cr 3+ ions in this model is approximately 2%, approximating the maximum doping normally encountered. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al 2O 3, in which 3 electrons leave each aluminium ion to join the regular octahedral group of six nearby O 2− ions in pure corundum this leaves all of the aluminium ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless, and transparent except for flaws.Ĭrystal structure of ruby showing the substitution of Al 3+ ions (blue) with Cr 3+ (red). Among the natural gems, only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Physical properties Crystal structure of rubies The world's most valuable ruby to be sold at auction is the Sunrise Ruby. Ruby is the traditional birthstone for July and is usually pinker than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated. The brightest and most valuable shade of red, called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. Some gemstones that are popularly or historically called rubies, such as the Black Prince's Ruby in the British Imperial State Crown, are actually spinels. The color of a ruby is due to the element chromium. The word ruby comes from ruber, Latin for red. Ruby is one of the traditional cardinal gems, alongside amethyst, sapphire, emerald, and diamond. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the most popular traditional jewelry gems and is very durable. Subadamantine, vitreous, pearly (on partings)Ī ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Orangy red through strongly purplish red. While the Mohs scale is largely considered near-accurate, one should treat the numbers more like ranks rather than a scale where the gap between each progressive number is equal.A ruby crystal from Dodoma Region, TanzaniaĪluminium oxide with chromium, Al 2O 3:Cr
Anything below 7, which is quartz-grade, is considered soft, as it cannot scratch glass easily.
Topaz is another gemstone that scores 8 on the Mohs scale. Beryl is one such constituent mineral of emeralds and aquamarine that scores 7.5 – 8 on the Mohs scale. Besides rubies and sapphires, only a few other gemstones are graded high for hardness on the Mohs scale. For every 14 parts of gold, when 10 parts of another metal are added, it is referred to as 14k gold jewelry. This innate softness is predominantly the reason why gold needs to be alloyed to enhance its strength and durability. Even gold, in its purest form of 24k gold, is only around 2.9 at best on the Mohs scale. Generally speaking, any gemstone or mineral that measures 7 or higher on the scale is considered hard and resilient enough for use in jewelry.
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