The term 'RED GEM' can refer to four different gemstones: RUBY, sard, GARNET, and CARNELIAN. RUBY is a deep red gemstone that is composed of the mineral corundum. Sard is a reddish-brown gemstone that is composed of the mineral chalcedony. GARNET is a deep red gemstone that is composed of the mineral pyrope. CARNELIAN is an orange-red gemstone that is composed of the mineral chalcedony. All four of these gemstones are commonly used in jewelry and other decorative items.