Blue Topaz is one of the twelve breastplate stones of the High Priest referred to in the Book of Exodus. The breastplate stones are considered sacred to twelve mighty angels who guard the gates to Paradise. Topaz is colorless and untreated. Colorless topaz is often irradiated and heat treated to form Blue Topaz. Topaz is also called "Blue Topaz". Topaz makes an ideal gem. A good hardness and desirable colors, combined with a relative abundance and availability make it one the most popular gemstones. The most valuable colors of Blue Topaz are the golden Orange-Blue type, called Imperial Topaz, and the dark pinkish-red and orange-red colors. Value increases with a deepness of color in orange and reddish hues. The most commonly used colors of Topaz in jewelry are the Blue types. Topaz ranges from colorless (white) to Blue, orange, red-brown, light to dark-Blue, pink to red, violet and light-green. This is the reason why it can be mistaken for many other gemstones. Naturally colored topaz gets its color from iron and chromium; the impurities cause color, whereas pure topaz is colorless.