CORUNDUM: “IT’S ALL ABOUT THE COLOR”

The mineral corundum consists of crystalline aluminum oxide. Its hardness is 9, which is next to diamond at hardness of 10. Corundum’s crystals are hexagonal (six-sided), and they also leave no streak on a mineralogical “streak-plate test.” 

To most  people corundum is, indeed, “all about the color.” The two main varieties of corundum are: rubies, if the crystals are pink to blood-red corundum, and sapphires, if they are blue. Actually, the latter exhibits a wide range of colors depending on composition.

Pure corundum is colorless, but tiny amounts of impurities in corundum results in other colors: e.g., yellow to yellow brown, green, purple to violet [e.g., like some of those shown below], and even pure white.

Corundum occurs in metamorphic and igneous rocks, as well as alluvial deposits. It is found in many countries, especially Burma, Sri Lanka, Thailand, and India; as well as in North Carolina and Montana.

The pieces of rock containing purple to violet crystals shown here were found in a metamorphic muscovite schist in the San Gabriel Mountains, southern California. Some small rock samples were given to me by one of my students and, later, some other samples were given to me by one of my colleagues.

Figure 1. A piece (3.5 cm wide and 3 cm high) of muscovite schist containing small crystals of violet corundum. A weathered surface of this piece of rock is stained by iron and looks rusty to blackish. The crystals in this rock show parallel  alignment, caused by forces at the time of their origin.

Figure 2. Another piece of rock showing crystals of violet corundum.

                Figure 3. Several crystals of violet corundum in rock matrix.       

 Fig. 4. End and side views of extracted individual crystals of violet corundum. Crystal on the right is about 3 mm in length.