Topaz is an aluminium silicate that contains fluorine and hydroxyl. In its pure form it is colourless (white). Impurities are what cause variations in colour. The use of topaz goes back to Egyptian times when the ancient Egyptians believed that yellow topaz received its golden hue from the Sun God, Ra. Some believe that "topaz" is a Middle English word, which was acquired from the Old French word "Topace" and Latin "Topazus", the root of which is in the Greek word "Topazios" or "Topazion" the ancient name of an island in The Red Sea where the ancient Greeks mined a yellow gem that they believed to be topaz. The name of the island means "to seek" in Greek. It could have been so named because it was difficult to find amongst the mist. This island is now known as "Zabargad" or "St John's Island", and it is thought that the gem mined by the ancient Greeks was actually "chrysolite". The ancient Greeks believed that topaz was a powerful stone that could increase the strength of the wearer and even provide invisibility in desperate times. Both the ancient Egyptians and the Romans associated topaz with the Sun God. Imperial topaz is the birthstone for those born in November and for those born under the zodiacal sign of Sagittarius. It is also the gemstone for the 23rd wedding anniversary. Blue topaz is the birthstone for December and is used to commemorate the 4th wedding anniversary.