Diamonds are the hardest naturally occurring mineral with a rating of 10 on the Mohs hardness scale. They are found in various countries around the world, primarily in stable parts of the Earth's crust. There are two main types of diamond deposits - primary sources which form deep underground and secondary sources formed by erosion near primary sources. The largest diamond producer currently is Australia, which produces about 1/3 of the world's diamonds annually. India was historically a major diamond producer but now has only one producing kimberlite pipe mine. Diamonds are evaluated based on the four C's - cut, color, clarity and carat weight.