Interesting - I too made a necklace with multicolor pink to blue chalcdony briolettes only to notice last week that the colors had indeed changed
Any agate or chalcedony that is bright pink, purple, blue, green, or
teal in color has been dyed. The organic dyes are light–sensitive
and will fade with time, especially under sunlight or fluorescent
light due to the UV content. The most stable color seems to be green.
Once the color has faded, nothing can be done except to try more dye.
Beads and cabs may be treated by a different process than slabs,
nodules, and bookends. There are methods of coloring agate and
chalcedony that use inorganic chemicals and these colors (such as
black onyx) shouldl be stable. Heat-treated agate should also be
stable.
Ray