Ruby is a deep pink to blood-red colored gemstone, a variety of the mineral corundum (aluminum oxide). It is considered one of the four precious stones, together with sapphire, emerald and diamond. Ruby is the red variety of corundum, all other colors of corundum being referred to as sapphire. The cost of natural rubies has risen significantly in recent years making them some of the most expensive gemstones on the market today. Nevertheless, rubies are still very popular in jewelry making and are used in rings, earrings, necklaces, bracelets and other pieces. Ruby is the birthstone for July and is also associated with the zodiac sign Cancer.