Diamonds are formed over a period of billion or more years deep within earths crust – about 150km (90 miles) deep. Within time, they are pushed to the surface by volcanoes. Most diamonds are found in volcanic rock, called Kimberlite, or in the sea after having been carried away by rivers when they were pushed to the surface.
A diamond is 58 times harder than corundum, the next hardest mineral on earth, out of which rubies and sapphires are formed. Only in the 15th century, it was discovered that the only way to cut diamonds was with other diamonds.