For my 18th birthday I received a silver bracelet, with what I believed were amethyst or similar stones set into it.

However, the purple stones appeared blue under the flourescent lighting in the biology labs, and under another type of light seemed clear white. Once under sunlight the stones again appeared their normal purple colour. There was no gradual fade of colour - moving from one light to a window yielded instant change - and under normal lightbulbs the stones also appeared purple, and the same with any light mixed with sunshine.

Is this to do with how the stones were cut, how they reflect the light, or is the stone itself responsible?

Thanks,

Eleanor