Corundum Occurrence and Use
Occurrence. Common as an accessory mineral in the metamorphic rocks, such as crystalline limestone, mica-schist, gneiss, etc. Found also as an original constituent of certain igneous rocks, usually those deficient in silica. Found sometimes in large masses, evidently the product of magmatic differentiation. Found frequently in crystals and rolled pebbles in detrital soil and stream sands, where it has been preserved through its hardness. Associated minerals are commonly chlorite micas, chysolite, serpentine, magnetite, spinel, cyanided, diaspore, etc.
Rubies asre found chiefly in Burmah, Siam and Ceylon. The most important locality in Burmah is near Mogok, 90 miles north of Mondalay. The stones are found here chiefly in the soil resulting from the decay of a metamorphosed limestone.. the have also been found in situ in the limestone. The rubies of Siam are found near Bangkok, on the Gulf of Siam, where they occur in a clay, derived from the decomposition of a basalt. The rubies of Ceylon are found with other gem stones in the stream gravels. A few rubies have been found in the gravels and in connection with the larger corundum deposits of North Carolina.
Sapphires are found associated with the rubies of Siam and Ceylon. The occur also in Kashmir, India. In the United States small sapphires of fine found in the river sands east of Helena when washing them for gold. They have since been found embedded in the rock of lamprophyre dikes. The rock is quarried and after exposure to the air for a time it gradually decomposes, setting the sapphires free. Sapphires are also found over an extensive area in central Queensland, Australia.
Massive corundum is found in the United States in various localities along the eastern edge of the Appalachian Mountains from North Carolina south. It has been Appalachian Mountains from North Carolina south. It has been extensively mined in southwestern North Carolina. It occur here in large masses lying at the edges of intruded masses of a chrysolite rock (dunite) and is thought to have been a separation from the original magma. Found as an original constituent of a chrysolite rock (dunite) and is thought to have been a separation from the original magma. Found as an original constituent of nepheline syenite in the Province of Ontario, Canada. At times the corundum is so abundant as to from more than 10 per cent of the rock mass.
The impure corundum, known as emery, is found in large quantities on Cape Emery on the island of Naxos and in various localities in Asia Minor. In the United States emery has been extensively mined at Chester, Massachusetts.
Artificial. Artificial corundum is now being made in the electrical furnaces at Niagara. Synthetic rubies and sapphires, colored with minute amounts of chromium, are made in considerable amount and in many cases it is difficult to distinguish them when cut from the natural occurring stones. Also small grains of the natural stone have been fused together in o larger masses, from which stones of two or three carts I n size can be cut. These are known as reconstructed rubies and sapphires.
Use. As a gem stone. The ruby at times yields the most valuable of gems; a stone of the deep red known as “pigeon’s blood” may bring large amounts. The blue sapphire is also valuable, and stones of other colors may command good prices.
Used also as an abrasive, either ground from the pure massive material, or in its impure form as emery. Artificial corundum and carborundum, which in composition is a carbide of silicon, are now manufactured on a large scale in electric furnaces and are being used in considerable amount as abrasives instead of the naturally occurring corundum.