Does anyone know what causes the blue color in blue Chalcedony?
I'm especially interested in colorant of the darker blue material
Natural blue color in Chalcedony is due to Tindal effect, which is
the same phenomena which makes our sky blue. Ray of light scatter on
their way through a medium and that gives off blue coloration.
However, if you observing darker shades of blue then the cause
probably is artificial treatment. Standard practice for coloring
Chalcedony blue is impregnation with potassium ferrocyanide
(prussian yellow) and with subsequent boiling in solution of
sulfate pentahydrate (blue vitriol of copper).