Sapphire is the non-red variety of corundum, the second hardest natural mineral known to mankind. The red variety of corundum is Ruby - all other colors are called sapphire, even pink.
Sapphires are well known among the general public as being blue, but can be nearly any color, even colorless. White (or colorless but massive) sapphire would more properly be called corundum. The blue color is by far the most popular color for sapphire but orange-pink, golden, white, and even black have generated much interest in the gem trade. Oriented rutile crystal inclusions cause a six-pointed-star light effect (called asterism) to form the popular Star Sapphire.
Many of the finest watches have crystal faces made of colorless artificial sapphire - these are extremely durable and scratch resistant.
One interesting bit of trivia - sapphire is aluminum oxide, and aluminum is a highly reactive metal. Any freshly exposed surface of aluminum is quickly oxidized to corundum (sapphire), so it can be said that your lawn chair (or screen door, or any other aluminum object) is coated with sapphire.