Why do some gemstones colour change under certain lights?

Any gem that changes colour in different lighting situations to the extent where the untrained eye can observe it is rare and highly prized by collectors.

Colour change gemstones display different colours according to changes in lighting.

Alexandrite “Emerald by day, ruby by night” from Russia, is the most famous colour change gemstone, and I’m always on the look-out for a fine specimen to add to my jewellery collection.

Colour Change Garnets exhibit a wide variety of colour changes. Some change from bronze in daylight to a rose pink under incandescent light. Others (like mine) change from a teal (blue-green) in fluorescent light to pinkish-purplish-red in incandescent light.

Colour change sapphires show equally dramatic colour changes, ranging from pink to purple and green to purple.

So why is it that so few gemstones exhibit dramatic colour changes, bearing in mind that nearly all gemstone colour depends on the nature of the illumination? The answer is that colour change gems have two, approximately equal-sized transmission windows. Only rare gems which formed with multiple light transmission windows can exhibit the ability to colour change

My Colour Change Garnet

A red gemstone appears red because it absorbs all frequencies of light except for red. A gemstone that absorbs all frequencies except for blue and red light will appear blue when the light is rich in blue wavelengths (e.g., in fluorescent light) and red when the light is rich in red wavelengths (e.g., in incandescent lighting).