Thus they probably formed at a depth of 100–200 km.Geologists believe that the ones we find must have been transported supersonically to the surface, in extremely violent eruptions through volcanic pipes.C at all if they really were over a billion years old, yet the radiocarbon lab reported that there was over 10 times the detection limit.
Newer, more accurate techniques use mass spectroscopy. It doesn't take much contamination to spoil a sample with near-zero quantity of C14. Another possible avenue is C13, which has a small but non-zero neutron absorption cross section.
Diamond is the hardest substance known, so its interior should be very resistant to contamination.
Diamond requires very high pressure to form—pressure found naturally on earth only deep below the surface.
The Problem: Accelerator mass spectrometry (AMS), a sensitive radiometric dating technique, is in some cases finding trace amounts of radioactive carbon-14 in coal deposits, amounts that seem to indicate an age of around 40,000 years.
Though this result is still too old to fit into any young-earth creationist chronology, it would also seem to represent a problem for the established geologic timescale, as conventional thought holds that coal deposits were largely if not entirely formed during the Carboniferous period approximately 300 million years ago.