Diamonds are the hardest naturally occurring substance on Earth, but are often most valued for their beauty. The highly cherished gemstones, which only form under conditions of extreme heat and pressure, were first discovered between 800 and 600 BC among river gravel in India and were not found in other parts of the world until the eighteenth century. Today, several different valuable deposits of diamonds are known, including those located in Brazil, South Africa, Australia, and Canada. Since the 1960s, synthetic diamonds have been produced commercially, though primarily as grit and small crystals for industrial purposes because the synthesis of gem quality diamonds is cost prohibitive.