Yellow sapphires belong to a group of colorful gemstones known as corundums, which is a crystallized form of aluminum oxide. Like other colors of fancy sapphires, the yellow is rather rare, and until recently, most of these more unusual colors were not well known among most people. Yellow occurs when a trace of iron is added to the aluminum oxide.
Sapphires are very hard stones, registering a 9 on the Mohs scale, and second only to the diamond in hardness. This makes the yellow sapphire not only a unique stone, but a very durable stone that can take everyday wear and retain its brilliance. As yellow sapphires increase in popularity, it is easier to find them in rings, pendants and earrings.