Natural onyx agate from Mali was investigated in an integrated
mineralogical and chemical study to reveal the origin of the unusual
black colouration. Detailed studies by polarizing microscopy, scanning
electron microscopy and micro-Raman spectroscopy showed that the colour
of the dark bands is related to the incorporation of small particles of
carbon (low-crystalline graphite) up to 200 nm in size into the
cryptocrystalline silica matrix. The dark bands have carbon contents of
1.88 wt.%. The location of the graphite particles is closely related to
the primary structural banding in the chalcedony. Cathodoluminescence
data shows that the banding is interrupted by small fissures containing
secondary hydrothermal quartz. The carbon isotope composition (delta
C-13 value of -31.1+/-0.2 parts per thousand) of the carbonaceous
material points to an organic precursor. Both the direct hydrothermal
formation of graphite from methane under elevated temperature and the
graphitization of organic precursors by secondary hydrothermal or
metamorphic overprint are possible explanations for the colour of the
dark bands. The graphitization of organic precursors results in an
intense electron spin resonance line at g(eff) = 2.0026.