Why are some gemstones called imperial?

Topaz is called imperial or precious, but only the deep yellow or brown variety. It also comes in other colors which are not as popular. Several gemstones come in more that one color, such as sapphire: A red sapphire is called a ruby and it is worth much more than a diamond of equal quality. Sapphires might be blue, orange, pink, yellow, or clear, but only the deepest blue is considered precious.