Just how extreme do the conditions need to be in order to form a diamond?

Just as you implied, naturally occuring diamonds need very extreme environmental conditions to enable them to form. Diamonds occur underground at a depth of between 100 km to 200 km below the Earth's surface. It's at these depths that the necessary pressure conditions of 45 to 60 kBars take place. Also, these depths also provide the appropriate temperature conditions that enable a diamond to form, ranging from 900 to 1300 degrees Celsius.

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The conditions needed to form a diamond are very simple: high pressure and high temperature (in the diamond industry this is often abbreviated as HPHT). For a naturally occurring diamond, these conditions are present deep underground and will take diamonds even millions of years to form. However, scientists have been able to mimic these extreme conditions in the laboratory far more efficiently. Under HPHT conditions, scientists are able to artificially synthesize diamonds in a matter of weeks.

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