Chrysoberyl produces the most celebrated colour-change gem there is, and the mechanism is worth setting out because colour change is frequently claimed and rarely genuine.
Alexandrite is chrysoberyl containing a small amount of chromium. The chromium produces an absorption band in the yellow part of the spectrum, so the stone transmits both the green and the red regions and absorbs between them. Which of the two dominates depends on what the light source supplies. Daylight and fluorescent light are rich in the shorter wavelengths and the stone reads green; candlelight and incandescent light are rich in the longer ones and it reads red. The stone is not changing. The light is, and the stone is unusually sensitive to the difference.
It was found in the Urals in the 1830s, in emerald workings, and named for the tsarevich who became Alexander II. The coincidence that green and red were the colours of imperial Russian military dress did the stone's reputation no harm at all. The Ural deposit was worked out, and later material has come mainly from Brazil, Sri Lanka and East Africa, generally with a weaker change.
The trade phrase, emerald by day and ruby by night, overstates it. A fine alexandrite shifts between a bluish green and a purplish red, and stones with a complete and strong change are among the most expensive gems by weight in existence. Most material sold as alexandrite is either synthetic or is colour-change sapphire or spinel, and the distinction requires testing.