Do diamonds actually form from coal getting compressed underground?

No. The coal-to-diamond idea is a myth. Both are mostly carbon, which is the only basis for the confusion, but their origins are unrelated. Three facts kill it. Timing: most diamonds are 1 to 3.5 billion years old (Precambrian), while land plants, the source of coal, appeared only about 450 million years ago, so diamonds predate coal. Depth: diamonds form 150 to 250 km deep in the mantle (a few superdeep ones at 660 to 800 km), whereas coal is near-surface rock rarely below 3 km, and crustal coal cannot be buried 150 km beneath a continent. Conditions: diamond needs about 900 to 1300 C and 45 to 60 kilobars, found in the mantle under stable cratons, not in coal seams. Diamond is also nearly pure carbon, while coal is full of hydrogen, oxygen, nitrogen, and sulfur. What diamonds actually form from is mantle carbon or carbon recycled into the mantle by plate subduction. Carbon isotopes show two types: peridotitic diamonds from primordial mantle carbon, and many eclogitic diamonds with light carbon signatures pointing to subducted surface organic matter and carbonate rocks. So diamond carbon can trace back to ancient life, but via subduction over geologic time, not by compressing coal. Diamonds also form at asteroid impacts and in meteorites, and synthetic diamonds are made from graphite or gas, never coal.</answer>

Sources:

• https://geology.com/articles/diamonds-from-coal/
• https://en.wikipedia.org/wiki/Diamond
• https://www.gia.edu/gems-gemology/fall-2023-diamond-reflections
• https://www.elementsmagazine.org/stable-isotopes-and-the-origin-of-diamond/
• https://www.smithsonianmag.com/science-nature/diamonds-unearthed-141629226/