Corundum is a very hard mineral that is commonly cut into gem stones. Corundum in it's natural state is clear, but it often collects impurities forming types such as Sapphire and Ruby. Corundum is the second-hardest hardness on the Mohs Scale.
Because of its hardness, corundum is often used industrially to create things such as abrasives and plastic making machine parts.
Rubies are highly valued for their exquisite look in jewelery. They are made of Aluminun oxide and other materials seep into Corundum. The formula for Rubies is (Al2O3::Cr). Rubies are also have a color of transparent red and are the birthstone of July.
Sapphire refers to the gem varieties with the minerals corundum, and aluminum oxide (Al2O3) when it a color other than red. A star sapphire is a semi-rare type of sapphire that shows a star-like phenomenon known as asterism. Star sapphires contain intersecting needle-like inclusions that cause the appearance of a six-rayed 'star' pattern when viewed with a single lightsource over the sapphire. Sapphire is also the birthstone of September.
"Corundum." Wikipedia. Wikimedia Foundation, 16 Dec. 2012. Web. 23 Dec. 2012.
Dutrow, Barbara. "Chapter 16." Mineral Science. By Cornelis Klein. 23RD ed. N.p.: n.p., n.d. 376-77. Print.