Diamonds form under severe conditions including tremendously high temperature and pressure. This only happens at a depth of 100 miles or more under the Earth’s surface; in the layer, we call the mantle. Simple carbon takes anywhere from 1 to 3 billion years to get converted into the raw diamonds we now mine. Volcanic and magmatic activity brings them high enough to dig out.
The sheer expanse of that duration is sobering, even humbling. The next time you are looking at a diamond and are about to dismiss it for a slight flaw you spotted inside it, remember that it has been around a very long time, and that, this in itself is reason enough to respect it.