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  This is a revised version of slide-illustrated presentions given in the past to gem and mineral clubs around the country.  It may be copied and distributed without restriction, except we request it is done not for profit (exception can be made for enhancing club treasuries).  It is not in PDF format, but we hope to have that or some other printer friendly version done soon.  However, on the computer most of the photos are thumbnailed, and clicking on them reveals higher resolution images.  



      Corundum is a special mineral, not only because of its economic importance (in gem form it is ruby or sapphire; and emery abrasive is a sub-gem form), but also because of its diversity of colors, forms, associations, and source localities which makes it uniquely attractive to collectors.  The specimens illustrated here are selected from our research/display collection to reflect this diversity.


     In its pure form, corundum is a transparent, colorless form of aluminum oxide, Al2O3.  It crystallizes in the hexagonal subsystem of the trigonal system.   Hexagonal prisms (such as the Kenyan sapphire on the left) and pyramidal or bipyramidal modifications (Burma, left and Sri Lanka, right) are characteristic forms.

     A most attractive bipyramid is this classic classic Sri Lankan "geuda" crystal on the left, which has been illustrated in pen and ink drawings in  European mineralogy books.  On the right are two Brazilian hexagonal prisms and a pyramidal cluster from the original Kashmir alluvial deposit - note white kaolin residue still attached.

     Hexagonal prismatic crystals occur in a variety of proportions: "elongated", "prismatic" (I know, redundant), "tabular", and "wafer", for example.  Those to the right also show some of the color variety.  They are (left to right) from  "Zoutspansburg", South Africa;  Karnataka, India;  Madagascar; Dat Taw Mine, Mogok Stone Tract, Burma;  and Tanzania.


     Some of the finest wafer crystals come from the United States; and one of the finest of these (left) is this 6.62 carat flawless gem from the Vortex Mine, Yogo Gulch, Montana.  The tabular ruby on the right is from the Dat Taw Mine, Mogok Stone Tract, Burma