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. Ruby crystals from India, though not of gem quality, are nevertheless famous for their beautiful colour, crystallization habit and size, and are used in jewelry-making for cutting necklace beads, cabochons, etc. This specimen stands out for the rutile crystal embedded in one of its facets, some quite rare.