Diamonds are one of our links between the world back then and the world now. We don't know if they truly are forever but we do know that they have been in existence a very long time.
Diamonds are made of carbon atoms that are formed into a tightly packed structure with extreme heat, pressure, and oxygen conditions. Most diamonds form in the earth's crust or mantle but some deeper down. Under the earth's crust they form as impact diamonds under the deep earth. due to mass pressure. The redox reaction of CO2 or Methane forms diamonds. The complex pattern of diamonds could take anywhere from 3 billion years to form in the earth's mantle and brought so the earth's surface through magma.