The Autumn birthstone is a combination of four gemstones: OPAL (4 letters), TOPAZ (5 letters), GARNET (6 letters) and APRIL (5 letters). OPAL is a precious stone that is known for its rainbow-like play of color. TOPAZ is a yellowish-brown gemstone that is often used in jewelry. GARNET is a deep red gemstone that is believed to bring good luck. APRIL is a diamond that is associated with innocence and purity. All four of these gemstones are associated with the season of autumn and make a beautiful combination for any jewelry piece.