Diamonds are not squished coal

Yami was asking about common geologic mythconceptions that piss us off, and the origin of diamonds is number one on my list. Diamonds are not former coal.

How do we know this? First, there is the chronology. Coal is the organic-rich remains of terrestrial plant matter, and this is younger than the oldest land plants, which are roughly 400 million years old. Pb isotope studies of diamonds show that they are generally between 2 and 2.8 billion years old, 5-7 times older than the oldest land plants. Thus diamonds were already collecting their pensions by the time the first coal beds were formed.

Secondly, they have different isotopic compositions. Plants, which fix CO₂ via photosynthesis, contain 2% less ¹³C than the Earth’s mantle, while most diamonds have mantle carbon isotopic composition. The light isotopic composition of coal is due to the preferential uptake of ¹²C over ¹³C during photosynthesis. Diamonds, not being related to coal, never photosynthesized, and do not show this effect.

Thirdly, metamorphic minerals have very different textures than hydrothermal ones. Diamonds are thought to precipitate from a CO₂ fluid when said fluid is reduced in the mantle. This allows big, low strain crystals to form. Solid state phase transitions generally involve lots of deformation and recrystallization. Were coal to be metamorphosed into diamond, it would probably form a diamond rock with micron-scale crystals. Impact diamonds have grainsizes that are micron to sub-micron, and the source of the carbon is difficult to determine. But they are tiny, and it is theoretically impossible for them to be more than a few carats (they need to be small enough to cool off before the shockwave dissipates, or they will revert to graphite).

Of course, it is theoretically possible to synthesize diamonds from coal in a lab. But this is unlikely to occur for several reasons. If the diamonds are being grown in a traditional metal catalyst belt apparatus, then a low sulphur carbon source should be used to prevent the nickel catalyst from being attacked. If coal was sulphur free, then trout in the Adirondacks would have nothing to complain about. In the case of Chemical Vapor Deposition diamond, a gaseous source- usually methane- is used. With either method, nitrogen from organic compounds in the coal would impart a yellow-green color in the diamond due to the absorption of the single N defect. So coal would be a poor source material for synthetic diamond production.

To summarize: Diamonds are too old to be squished coal, and even if they weren’t they contain the wrong sort of carbon and form through different processes. Furthermore, coal is a poor choice of precursor for synthetic diamond production, as spectroscopic graphite is best for metal catalyst diamonds, and methane is preferred for CVD. If you really wanted a diamond made from coal it could be done, but you might as well make diamonds from your dead granny and exhaust fumes.