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or Adamantine Spar Corundum

qv, deposits and °oriental

CORUNDUM, or ADAMANTINE SPAR, a native oxide of aluminum, A1,O., crystallizing in the rhombohedral system, and also oecurti* massive. Its hardness is nine, audits specific gravity about four. It is adamantine or vitreous in lustre, and very variable in color. ' Three varieties are commonly recognized. Of these the first is known as sapphire, and includes those specimens that are used as gems (q.v.), The typical sapphire (q.v.) is blue, the red kind being known as °oriental the yellow as aoriental topaz,>> the green as °oriental emerald,o. and the purple or violet- as °oriental ainethYst.* The colors of these gems are due to the •res ence of traces of metallic oxides. The second principal variety- of the mineral is that which is known in the arts simply as and is used as an abrasive (q.v.). It in cludes the less transparent varieties of blue, brown, black, gray or white culors.. It is 641er crystallized or granular, or in Masses showing, distinct parting. The third variety, eemery,)' is

not pure, but is an intimate mixture of corun dum with magnetite or hematite. It has long been one of the most important abrasives (q.v.),, but it is now being superseded by the greatly superior pure corundum. This usually occurs, in crystalline rocks such as granite, gneiss, nepheline-syenite, granular limestone, also chlo rite and mica-slate. Sapphire and ruby (qq.v.) are usually in alluvial deposits and in the beds of rivers. The Canadian corundum deposits, discovered a few years ago, are the largest and, most important known. Corundum is also found in many other localities, notably in North Carolina, Georgia, Montana and India. The commercially valuable deposits in the United States occur chiefly as ifi Okra basic igneous rock (dunite) in North Carolina, and Georgia, while the Canadian deposits as pegmatite dikes (q.v.).