The sapphire is a corundum, an aluminum oxide with a trigonal crystal structure, in the same family as the ruby. The only difference between a ruby and a sapphire is simply the color. A red corundum is a ruby. Other colored corundums are called sapphires, which come in many colors, the most well known being blue. Because sapphires are available in so many colors, they are the most important and versatile of all the gemstones. Rubies and sapphires are said to be prized just under the level of diamonds because of their hardness. Diamonds are listed as a ten in terms of hardness, sapphires as a nine. The attribute of hardness of the sapphire makes it a perfect choice for jewelry that needs to stand up to everyday wear, such as in rings or bracelets.