NASA determined that they are made of an iron - rich mineral called haematite and others pointed out that similar marbles were to be found on Earth. (newscientist.com)
Once a ball starts to form, it builds up layer by layer as the iron precipitates out of the water to form the mineral haematite. (newscientist.com)
Simandou is a major new, high quality haematite deposit (with a targeted grade of 65 per cent iron) in proximity to the Atlantic Basin and the Middle East. (politics.co.uk)