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.

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.

Corundum sapphire crystals were recently discovered at the Blu Jay Mine near Revelstoke, and are the crystallized form of aluminum oxide. Corundum has been noted in several other areas of B.C., starting with a reported placer occurrence in 1915 of a green, transparent 7mm sapphire on the Pend d’Oreille River in the West Kootenay District. Corundum has been reported in plumasite dikes occurring in the Polaris Ultramafic Complex in the Omineca area and in the Ice River Complex near Field, and in the Prince Rupert area within high-grade metamorphic gneisses. It is a minor component of the metamorphic aureole around porphyry copper-gold-silver deposits at the Equity Silver Mine, Limonite Creek property, Taylor-Windfall Mine, and the Empress property.

Blue sapphire has been noted in samples taken from the Mark diatreme in the Golden area, and small placer rubies have been found in the Atlin and Tulameen area. Corundum and cordierite occur together in a garnet-anthophyllite layer several kilometres in length in the Thor-Odin Complex, north of the Valhalla Complex. Corundum has been found in correlative metamorphic rocks south of Frenchman Cap Dome near Revelstoke. The sapphires found on the Slocan Gemstone Property are with few exceptions the variety known as star sapphire. These stones often display a 6-rayed asterism due to rutile inclusions when cut in a cabochon, and most often they have a semi-translucent grey-blue to bronze-black color. Sapphire mineralization on the property is known from the Blu Starr, Blu Moon, Sapphire Hill and New Star areas. The Blu Starr, New Star and Sapphire Hill sapphire occurrences appear to be partially stratigraphically controlled, forming within and proximal to plumasite dikes intruding shallow-dipping mafic gneiss. This represents an exposed and inferred mineralized zone with an outcrop distance of more than 6 kilometers, and it is still open to expansion. The Blu Moon sapphire deposit is unique in that it occurs disseminated within sodalite-bearing nepheline syenite proximal to a feldspar-amphibole dike that is itself unmineralized. A second unique kind of sapphire deposit occurs on the adjoining Spin property in the Tedesco area, where small blue sapphire crystals occur intergrown with blue-green spinel crystals in green mica filled vugs within a highly metamorphosed marble.

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