Diamond is a form of Carbon where due to the high temperatures and pressures exerted on carbon for up to 3.3 billion years causes the atoms to be arranged in a particular structure we know and love as diamond. Most of the earth's diamond as in actual fact been created at between 140 and 190 km depth inside the planet's core, where the required temperatures and pressures exist. These diamonds take between 1 to 3.3 billion years to form, and it is thought that most of the diamonds we mine today are either then brought to the surface by volcanoes or by asteroid impact many millions of years ago.
Diamonds rate a 10 on the Mohs hardness scale making it the hardest natural substance on the planet and come in many colours ranging from colourless to yellow, gray, blue, pink, purple and red depending upon the impurities within the diamond.
Diamonds will initially look something like these pictures below.
The first thing is to cut the diamond and then it is polished - as in these two photographs below
Here you can see a diamond which has been cut with a diamond saw
After cutting and polishing, it will look more like the diamonds we know