Banagher, about temperatures.....and some other things.
The temperature of lava.....normal lava temp is 750 deg C, the hottest is 1250 deg C.
How and where are diamonds formed? Diamonds form between 120-200 kms or 75-120 miles below the earth's surface where the pressure is around 5 gigapascals, and the temperature around 900 to 1300 deg Centigrade. According to geologists the first delivery of diamonds was somewhere around 2.5 billion years ago and the most recent was 45 million years ago. That is a long time, my friend!
According to science , the carbon that makes diamonds, comes from the melting of pre-existing rocks in the Earth's upper mantle. There is an abundance of carbon atoms in the mantle. Temperature changes in the upper mantle forces the carbon atoms to go deeper where it melts and finally becomes new rock, when the temperature reduces. If other conditions like pressure and chemistry is right then the carbon atoms in the melting crustal rock bond to build diamond crystals.
There is no guarantee that these carbon atoms will turn into diamonds. If the temperature rises or the pressure drops then the diamond crystals may MELT partially or totally DISSOLVE. Even if they do form, it takes thousands of years for those diamonds to come anywhere near the surface.
The diamonds that we see at the surface are ones then that are brought to the surface by a very deep-seated volcanic eruption. It's a very special kind of eruption, thought to be quite violent, that occurred a long time ago in the Earth's history. We haven't seen such eruptions in recent times. They were probably at a time when the earth was hotter, and that's probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth. When the eruption reached the surface it built up a mound of volcanic material that eventually cooled, and the diamonds are contained within that. These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds.
Annealing of gem-quality diamonds at very high pressures (above 5 GPa) and temperatures (above ~1800°C) can produce significant changes in their color. Treatment under these high-pressure–high-temperature (HPHT) conditions affects certain optically active defects and their absorptions in the visible spectrum. In the jewelry industry, laboratory-treated diamonds are valued much less than those of natural colour.
Artificial diamonds have been produced since the 1950's in processes that simulate the extreme conditions of temperature and pressure in the earth's crust where natural diamonds are formed. Just as in Nature, however, growing a large crystal takes geological times, so only small crystallites can be produced economically. The nonequilibrium techniques being developed in my lab (Peter Taborek) are very different: they utilize gas or plasma at very low pressures and high velocities, which leads to high growth rates. One particularly efficient technique developed as part of Derrek Russell's PhD thesis is a plasma jet. In this device, hydrogen gas with a small amount of hydrocarbon added is heated to temperatures of 8000 degrees Kelvin, hotter than the surface of the sun. The resulting plasma is injected into a vacuum chamber at speeds of Mach 4. The plasma is quenched onto a metal plate where the temperature drops from 8000 degrees K to 1200 degrees K within a few microseconds. If conditions are just right, a diamond film 3 inches across and nearly a millimeter thick can be grown on the plate. This process converts methane into diamond simply by heating and cooling the gas, a trick that would make even the alchemists jealous.
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