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 topaz are the golden orange-yellow type and the dark pinkish-red and orange-red colors, sometimes called Imperial Topaz. 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. It was not until this past century that blue topaz became widespread on the gem market since virtually all blue topaz is irradiated and heat treated.