Corundum was chosen by the geologist Friedrich Mohs to represent grade 9 in his famous scale first published in 1825 and which is still valid today. It is second hardest material on Earth. All crundums of gem quality, except the red ones (ruby) and the blue ones (sapphire) are named sapphire plus its colour, except for the blue ones, which are just called sapphire. The colourless ones are also called leukosapphire from the Greek leukos = white.