
Sapphire is the blue variety of corundum, the second hardest natural mineral known. Red forms of corundum are known as ruby. Sapphires are found in alluvial gravels in river beds. They settle in river beds after having been eroded by weather, dislodged from their original locations, and swept downstream.
One of the largest sapphires ever discovered was the Star of India, extracted from the Earth in Sri Lanka. This amazing specimen weighs 563 carats (4 ounces) and is housed in the New York Museum of Natural History.
Sapphire is generally synonymous with blue, although the gemstone may also be orange-pink, golden, white, or even black. Classic blue sapphire is colored by impurities of iron and titanium that are lodged within the body of the corundum crystal. Oriented rutile inclusions cause a six-pointed effect known as asterism to form the popular “star sapphire” variety.

Sapphires are hard-wearing and highly resistant to heat and electricity. The finest specimens are cut in a variety of styles for jewelry. Even inferior stones are used for clock mechanisms, watches, electrical measuring instruments, and even in the rolling nibs of expensive pens.
Synthetic sapphires are made by a flame-infusion technique in which powdered corundum is melted at high temperatures with iron and titanium. Among the natural gemstones that resemble sapphire are blue spinel and blue tourmaline. Blue tanzanite also looks similar, but it is softer, scoring only 6.5 to y on Mohs scale of hardness. Genuine sapphire has a Mohs rating of 9.
