Diamonds mined from the earth are believed to have formed around 1 to 3 billion years ago, approximately 90 miles below the surface. It was all very intense down there. Carbon dioxide was exposed to extreme temperatures (1093+°C) and extreme pressure, then the diamonds were moved from the core to its crust by a series of volcanic explosions.
Lab grown diamonds however, have been manufactured using conditions that mimic the natural process, so the carbon is still subjected to high temperatures and high pressure but in a controlled environment so there are no unexpected pyrotechnics and explosions involved.