Although the concentrations are small - a fraction of a gram of gold in every ton of coal - the scientists believe it may one day prove commercially viable to mine coal for its gold content.
The average concentration of gold was 0.04 grams per ton of South Wales coal but the two researchers, Rod Gayer and Professor David Rickard, at the University of Wales, Cardiff, found gold concentrations as high as 4.42 grams (1.55oz) per ton in the ash left over from burning coal.
Professor Rickard said higher concentrations may be possible and because coal is such a huge geological deposit, the prospect of extracting it commercially 'is worthwhile looking at a little more'.
Writing in today's Nature, the researchers say it is the first time that gold has been found in coal. 'One of the amazing things is that we can see the gold under a microscope and take pictures of it,' Professor Rickard said yesterday.
British Coal's stockpile of coal is about 13 million tons, worth about pounds 351m at current prices. Assuming a gold content of about the average found in Welsh coal, this means the stockpile contains about half a ton of gold, worth nearly pounds 5m.
Extraction would not be economic on these figures, but Professor Rickard says that the advantage of coal is that it can be burned first and then the ash could be processed relatively cheaply to extract its gold.
'We may not have found the highest value for the gold concentration in coal and when you burn coal you end up with a higher concentration still in the ash.'
Where the gold came from is a mystery. Professor Rickard suggested that the ancient plants that were ultimately turned into coal probably concentrated the gold that was initially present as a watery solution.