geology
The colour in Agate is similar to the colour in stained glass. The small amount
of a metal mixed in with the clear silicate mineral gives it colour. Different
metals make different colours. The colours that chemicals like copper sulfate
have in solution are similar but the colour that a metal ion gives to a
solution will be differnt from the colour that the same metal gives to agate.
Colour is caused by the atoms and the bonds between atoms absorbing some
wavelengths of light and not others. I hope this is a start please ask for
clarification if needed.