The article below is a version of a slide-illustated talk periodically given at gem and mineral club meetings.  Most of the photos are thumbnailed, so clicking on them will reveal the full-resolution images behind them.  


     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. 

California,  Nepal, Burma, Russia

     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 or pyramids and modifications are characteristic forms.  It is extremely hard (9 on the Mohs scale) and rather dense (specific gravity about 4.00).

Prism (Kenya)

Prisms (Brazil)

Pyramid (Burma)



     Perhaps the most attractive are the bipyramids such as the classic Sri Lankan "ruby" or the "geuda" crystal on the left.   The cluster of bipyramidal crystals on the near right is from the classic Kashmir alluvial deposit (note the white kaolin attached).   Pure corundum is colorless and tansparent.  The far right stone and the one above it are gemmy "white sapphires", sometimes called "poor man's diamonds".  

Bipyramid (Sri Lanka)

Bipyramid (Sri Lanka)

Bipyramid (Kashmir)

Bitapered prism (Burma)


     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, India.  Madagascar;  Burma,  and Tanzania.  The two tabular crystals on the rught are from Montana and Burma




       Traces of other metallic ("transition") elements replacing aluminum in the crystal lattice can produce just about any color of the rainbow.   

The "pigeon-blood red" of the finest rubies is usually caused by chromium.  Mineralogically speaking, it may be said that any chromium-rich corundum with reddish color is ruby (though jewelers may tend to draw the line with less tolerance).

Ugandan Road Kill Pigeon

Star Ruby Crystal (india)

Large gems (Burma)



    The blue and fancy-colored sapphires owe their colors mainly to the presence of traces of iron and titanium.  See the article on origin of color in the "Science" section for the technicalities (article in preparation).

Blue (Viet Nam)

Green (Russia)

Brown (South Africa)

Purple (Pakistan)

       Corundum can occur as single crystals, twins, clusters, and even in massive forms.   

     Crystals can have lateral faces of different forms.  Below are examples of twins, clusters, and massive specimens, and even a rare corundum of hydrothermal origin!.

Smooth (Madagascar)

Striated (Viet Nam)

Layered (India)

Different (India)


Interpenetrant twins (India)

Contact twins (Afghanistan)

Tabular cluster (Burma)

Spicular cluster (Nepal)

Massive (North Carolina)

Hydrothermal (Japan)

       There are many different rocks and minerals that may be found with corundum in matrix specimens.  Perhaps the most common is calcite.  Here are some of the other possibilities.  The last specimen shows ruby which formed in a clay bed as it petrified - with opalized snail shells!!  

Calcite (Viet Nam)

Diopside, calcite (Tanzania)

Smaragdite (North Carolina

Syenite (Viet Nam)

Petrified clay (Kenya)


Gneiss (India)

Gneiss (Norway)

Zoisite (Tanzania

Zoisite (India)

Fuchsite (Wyoming)

Schorl (South Carolina)

     Spinel may occur with corundum.  The three specimens on the left are spinel octahedra with corundum inclusions.  Note in the third that there are rubies in the center and lavender sapphires lining the edge of the triangular face closest to the camera.  The crystal on the right is Burmese, and is a unique " corundum after spinel" pseudomorph.