alexandrite

noun

1. (countable, uncountable)A form of chrysoberyl that displays a colour change dependent upon the light source, along with strong pleochroism.

   “[…] the body of each high seat was a single jewel of monstrous size: the left-hand seat a black opal, asparkle with steel-blue fire, the next a fire-opal, as it were a burning coal, the third seat an alexandrite, purple like wine by night but deep sea-green by day.”