Purple corundum, or sapphire of amethystine tint, is called Oriental amethyst, but this expression is often applied by jewellers to fine examples of the ordinary amethystine quartz, even when not derived from Eastern sources.
Corundum indeed is abundantly met with in the eastern half of Liberia.
The mining of corundum was begun at Corundum Hill in Macon county in 1871, and from 1880 to 1902 the output was considerable, but with the discovery of the Canadian corundum Scale, 1:2,500,000 English Miles 20 30 4 0 So County Seats e County Boundaries Railitlays Canals Swamps deposits the importance of those of North Carolina greatly declined.
In emery, magnetite in a granular form is largely associated with the corundum; and in certain kinds of mica magnetite occurs as thin dendritic enclosures.
SAPPHIRE, 1 a blue transparent variety of corundum, or native alumina, much valued as a gem-stone.
The great corundum deposits ofCorundumHill,Macon county,N.C.,have yielded good sapphires, and they are found also at Cowee Creek in the same county.
Although the term sapphire is primarily applied to blue corundum, it is often used in a general sense so as to include all corundum of gem quality, regardless of colour.
Hence clear colourless corundum is known as white sapphire or "leucosapphire."
Yellow corundum is not uncommon in Ceylon and is termed yellow sapphire or "oriental topaz," the prefix "oriental" being often applied to corundum.
(For figure of crystal of sapphire see Corundum and for artificial sapphire see GEM, § Artificial.) The so-called "Hope sapphires" of trade have been shown to be artificial blue spinets, coloured by cobalt.
The central space of the furnace was filled with a mixture of corundum, coarsely-powdered charcoal and copper; and an iron lid lined with firebrick was luted in its place to exclude air.
Aluminium oxide or alumina, Al 2 0 3, occurs in nature as the mineral corundum, notable for its hardness and abrasive power (see Emery), and in well-crystallized forms it constitutes, when coloured by various metallic oxides, the gem-stones, sapphire, oriental topaz, oriental amethyst and oriental emerald.
This powder, provided that it has not been too' strongly ignited, is soluble in strong acids; by ignition it becomes denser and nearly as hard as corundum; it fuses in the oxyhydrogen flame or electric arc, and on cooling it assumes a crystalline form closely resembling the mineral species.
The name 'Aba as, " the invincible," was probably applied by the Greeks to hard metals, and thence to corundum (emery) and other hard stones.
It is the hardest known substance (though tantalum, or an alloy of tantalum now competes with it) and is chosen as io in the mineralogist's scale of hardness; but the difference in hardness between diamond (io) and corundum (9) is really greater than that between corundum (9) and talc (1); there is a difference in the hardness of the different faces; the Borneo stones are also said to be harder than those of Australia, and the Australian harder than the African, but this is by no means certain.
African locality must be mentioned.; considerable finds were reported in 1905 and 1906 from gravels at Somabula near Gwelo in Rhodesia where the diamond is associated with chrysoberyl, corundum (both sapphire and ruby), topaz, garnet, ilmenite, staurolite, rutile, with pebbles of quartz, granite, vIII.
Chaper found diamond with corundum in a decomposed red pegmatite vein in gneiss.
From these rocks in the Ottawa valley are quarried or mined granite, marble, magnificent blue sodalite, felspar, talc, actinolite, mica, apatite, graphite and corundum; the latter mineral, which occurs on a larger scale here than elsewhere, is rapidly replacing emery as an abrasive.
Among the less important metals and minerals which are also mined, is corundum of especial purity.
Copper ore is found in many tracts throughout India, plumbago in Madras, and corundum in southern India.
Haematite crystallizes in the rhombohedral system, and is isomorphous with corundum (Al203).