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[R.U. Sirius]

I've been told that amethyst colour may bleach in sunlight, and of course there are examples of much more fragile minerals - but I would have never guessed that a typical "blue" Brazilian banded agate specimen could change so much after three years of storage indoors.  Arguably, this is due to drying out, as in loss of water molecules that were absorbed deep into the microcrystalline structure.

The image is from a 2020 review paper on the structure and formation of agates by Jens Götze et al. It is freely accessible here. It covers many topics and theories, and I highly recommend it, but I am hoping to keep the discussion in this thread focused on this example of unstable appearance of polished agate specimens.

Have any of you observed any significant changes in colours of your agate specimens?

[vitzitziltecpatl]

Okay - I'm hooked. Downloaded the paper.

Can't think of any specimens here that have faded like that, but will look at pieces now with this in mind.

[peruano]

Was it dyed to begin with?

[lithicbeads]

Many types of rock  change ass they dehydrate. Agates are particularly prone due to the porosity of many of the layers , a trait that makes them so easy to dye.

[R.U. Sirius]

Peruano - no, it was cut and polished. I wonder if ageing of the cutting oil residue within the pores may also have something to do with it.

In my small collection a few agates have developed cracks that were not there few years prior, this also may have to do with drying out. In Ethiopian and Spencer opals this is much more obvious.