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Corundum
According to the University of Nebraska-Lincoln, sapphires and rubies are both made of the mineral corundum, which is aluminum oxide. All specimens of corundum that are not red are sapphires. Next to a diamond, this mineral is the hardest thing on Earth ranking at an eight on the Moh's scale of hardness. The blue sapphire version of corundum is slightly harder and denser than the other varieties.
Color
The presence of titanium and iron in different combinations is what gives corundum its different colors. One characteristic that sets sapphires and rubies apart is the variance in color in sapphires. Not only do sapphires exhibit different dominant colors like blue, green, yellow, pink and white but they also exhibit color variations within each stone. A sapphires color is dependent upon the temperature it reaches in the earth. Commercially, sapphires are treated to purify or further diversify their color.
Star Sapphires
Gems that have unique asterisms beneath the surface of the stone are called star sapphires. The star appears when light reflects off of the inner basal planes, or layers of the crystal. The light appears in a star shaped pattern that extends from the center of the sapphire out to the edges. A perfect, fully extended six-rayed star is the most valuable and desirable star sapphire. Some asterisms are sharper in image than others are. The sharper image is more desirable.
Occurrences
Kashmir, India, has produced the world's most valuable sapphires. Though a Kashmir sapphire pendant sold at Christie's auction in 2007 for more than three million dollars, the region's mines have been abandon for years. The Himalayas are a prime basalt riddled landscape for sapphires, found throughout the region. In the United States, most sapphires come from Montana. North Carolina has also produces some worthy sapphires, according to the University of Nebraska-Lincoln.