Ruby stone engagement rings are the perfect gemstone alternative to the ever-popular diamond engagement ring. This fiery red jewel is an eternal symbol of romance and passion. Along with the sapphire, the ruby is part of the corundum gemstone family. It is also a 9 on the Mohs hardness scale (diamonds are rated 10). The Gemological Institute of America classifies ruby as having a “dominant red hue,” a distinction that separates rubies from pink sapphires and other similar-looking stones.