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 corundums of gem quality, except the red ones (ruby) and the blue ones (sapphire) are named sapphire plus its colour, in this case, green sapphire. Green sapphires with the most intense colour stem from Australia. The specimens from Sri Lanka, such as this one, have a slight greenish hue.