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I haven't followed this string carefully and maybe someone else has
mentioned this, but the gem ammolite now on the market is not opalised.
In certain deposits, mainly in Canada, fossilized ammonite shell has
been replaced by flat platelets of aragonite (calcium carbonate),
according to scanning electron microscope studies by Henry L. Barwood,
Ph.D. The arrangement of these platelets causes "interference fire"
instead of diffraction colors as with precious opal, he says. When cut
properly (some treatment is required, I understand), ammolite exhibits a
brilliant play of color somewhat like opal. The difference, according
to Barwood, and I quote him, is that ammolite "tends to run to 'second
order' reds and greens; opal typically exhibits 'primary' or first-order
spectral colors of greater purity and intensity."
Just thought you'd like to know in case you're on "Jeopardy" when the
question comes up.