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Corundum
Corundum is an abrasive aluminum oxide mineral found in igneous and metamorphic rocks. Both rubies and sapphires are fully crystallized forms of corundum. Crystallized corundum that forms with traces of iron and titanium turns a deep blue color. This corundum is called sapphire. Other sapphires include pink, green and yellow colors that are due to different trace metals found in the parent mineral.
Hardness
Sapphire is a very hard material. It has a rating of nine on Mohs scale of mineral hardness. A diamond is the hardest mineral on earth with a Mohs rating of 10. There are no minerals between corundum and diamond on the scale and so corundum is the world's second hardest mineral. Because of this quality, corundum is used as an effective abrasive in commercial products like sand paper, cutting tools and polishing compounds.
Structure
Sapphires exhibit a geological structure called hexagonal system. A crystal structure refers to lines of weakness along which the stone will break naturally. A hexagonal crystal has six sides with four axes that run along central lines in the stone. Other gems that exhibit a hexagonal structure include emeralds, aquamarine, rubies, tourmaline and quartz.
Gems
According to Mine Links, sapphires often exhibit irregularities in color. Some gems are heated to create uniformity. The blue color fades when the stone is heated so dark patches can be lightened to add value to the gem. Should a pattern form in the irregularities of a sapphire, the gem may be cut to highlight these characteristics. Darker gems are more highly valued as long as the dark color does not interfere with the stones translucent clarity.