Diamonds were made under intense pressure and heat far below the Earth's surface.
The diamonds we see today in jewellery and used for industrial purposes are millions of years old. They were bought to the Earth's surface by volcanic forces and we can then find them in, the long dead, volcanoes. This is where the vast majority of diamonds are found. We can then track down the diamonds in the volcanic area and then mine them to remove them from the ground.
Over millions of years, diamonds can be eroded away, for example, by wind and water and move away from the volcanic areas and end up in places like river beds. So we can also mine old river beds to retrieve the diamonds.
Diamonds have been found all over the world, with diamond mines in Australia, Canada, Brazil, Africa, Russia and India at different times through history.
Many diamonds found at the same mine will share similar characteristics. For example, The Argyle Mine in Australia produces approximately 95% of the worlds pink diamonds. Diamonds found in a specific mine in Botswana tend to be large and of a very high quality (and very rare!). Diamonds found in Russia tend to be smaller and of poorer quality therefore, are generally used in industry rather than for jewellery.
Diamonds take millions of years to form and then to reach the surface of the Earth, and then it takes a long time for us to track them down and remove them from the ground. So diamonds you see in jewellery today have taken a very long time to get there!