Ruby is a pink to blood-red colored gemstone of the mineral corundum. All other varieties of corundum are called sapphires. Ruby is one of the four "precious" gemstones, along with Diamond, Emerald, and Sapphire. The Ruby is a stone of nobility, considered the most magnificent of all gems, the queen of stones, and the stone of kings. Its value exceeded that of diamonds, and the Chinese Emperor Kublai Khan was said to have offered an entire city in exchange for a sizable Ruby. Ruby has always been a talisman of passion, protection, and prosperity. It was worn as an amulet to ward off plague and pestilence and warned it's wearer of impending danger. Ruby actively stimulates the Base Chakra, increasing vitality and chi. It promotes a clear mind, increased concentration and motivation, and brings a sense of power to the wearer. Ruby has been associated with love, especially faithful passionate commitment and closeness. In antiquity, Rubies were considered to be perfect wedding stones.