Some topaz has a metallic oxide coating that gives the stone a colorful iridescent shine. Other topazes are entirely dull in color. Topaz is a metallic gemstone with a metallic lustre. It is often white, but it can also be yellow, red, blue, green, or black. The most valuable topaz is a deep violet-blue called imperial topaz. Less valuable stones are called citrine, amethyst, and chrysoprase. Metals include gold, silver, platinum, copper, and zinc. Gemstones include agate, amber, emerald, jade, onyx, opal, sapphire, spinel, tanzanite, and tourmaline. Topaz is the birthstone for April. It is believed that the name "topaz" comes from the Greek word taphos, which means burial. This is because the stone was used as an amulet to protect against death. There are several varieties of topaz. Imperial topaz is the rarest and most expensive variety. It contains high levels of aluminum oxide (Al 2 O 3), which gives the stone its purple color.