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Fading agate colours


Can anyone tell me why some of my coloured agate slices have faded
and others have not?

I had a small selection of coloured agate pieces that I used for
some pendants a couple of years ago. The ones I didn’t use at the
time have been kept together in a plastic tray in one of those
stacking storage boxes so they have been kept in the same conditions
away from light. They are mostly purple and a pretty turquoise/green.
Some have faded to almost gray and others have remained strongly
coloured. (I have no idea how they are coloured in the first place
but I realized they weren’t natural.) The brown and orangey coloured
pieces haven’t changed at all - I presume these are natural colours?

Can anyone advise me why some have changed and consequently how they
should be kept to avoid this?

Thanks for your help

Most of the brightly colored agate from Brazil (except green) will
fade on exposure to light since the dyes they use are apparently not

The brown, black and gray colors are natural and are stable. The
orange carnelian-like color is produced by heat treatment and is also
stable. Dyes used in other countries may be more or less stable. I do
not know of any way to prevent this fading but blocking UV exposure
should slow it down.

Ray Prater