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This is Alexandrite, it’s also called “emerald by day, ruby by night”

It changes colour based on whether the light source is from the sun or from a candle.

It does this because Alexandrite strongly absorbs yellow light due to chromium ions in its crystal structure, leaving the other colours behind. Light from the Sun emits all colours, but it peaks in the green, and our eyes are most sensitive to green, so in Sunlight Alexandrite is green.

Incandescent lights are things like candles and filament light bulbs. They also emit all colours of light, but they peak far, far into the red, so there’s not nearly as much green or blue, so under those, Alexandrite is red. 

Gemstones are awesome.


I’ve been learning about gemstones in my GIA classes so you guys get to learn more about Alexandrite! It was discovered in Russia in the early-mid 1800’s and is found in a lot of antique Russian jewelry. Sadly, the mines in Russia have been largely exhausted, and with them the supply of the best quality Alexandrite. New sources in Brazil are not as fine and tend to shine brown in incandescent light rather than red, but they do display cat’s-eye effects, making them valuable in their own right.

Another issue is that of sources of incandescent light. In an effort to encourage energy efficiency many governments across the world are forcing the phase-out of incandescent light bulbs. Since most people don’t light their homes by candlight, increasingly most people will find that their Alexandrite is green by both day AND night.


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