If not, is there a blue stone that can fade in the dark, and looks
like blue chalcedony?
This is just a wild shot in the dark set off by associating a couple
of things I know to be true. There are transparent-to-translucent
common opals (no play of color) that occur in wonderful blue hues.
I’ve mined them myself. Opal contains varying amounts of water, with
stable opals containing about 6% according to opal expert Len Cram.
Most of the blue opal I’ve seen is more hydrated than that, drying
out and turning opaque white in a period of weeks or months when
exposed to the air.
A safe deposit box is the world’s worst place to store opal of any
kind. The air inside is carefully controlled to eliminate humidity in
order to protect currency, stock certificates, etc. from
moisture-caused deterioration. Even stable opal needs a certain
amount of atmospheric moisture to allow rehydration as it slowly
loses water day to day. But keeping it in a bank box not only
deprives it of its water source but accelerates dehydration. I’ve
heard many sad stories of valuable opals crazing after being stored
I’ve seen stable blue opals, some even with play of color. It’s
possible they were used in your piece. I’m assuming the stones are
cabochons because blue chalcedony isn’t normally faceted. Just one
possibility to check out.