Alexandrite

If you love magic, especially the magic of science, you'll love Alexandrite, the colour-change gem.  Outside in daylight, it is a cool bluish mossy green. Inside in lamplight, it is a red gem, with a warm raspberry tone.  You can watch it flick back and forth by switching from fluorescent to incandescent light.

Alexandrite is a gem variety of the mineral chrysoberyl discovered in 1830 in Czarist Russia.  Since the old Russian imperial colours are red and green it was named after Czar Alexander II on the occasion of his coming of age.

Colour Change

Most gems transmit and absorb light throughout the visible spectrum and we interpret the mixture of the transmitted wavelengths as the gem's color. Alexandrite transmits light only in two discrete windows of the spectrum, in the blue-green and red regions. The rest of the spectrum is absorbed.

When viewed under light sources active in those particular regions of transmission, you see one of the two colours.  In daylight, or in artificial light that simulates it, like fluorescent light, light waves in the green region predominate. 

In candlelight, or artificial light that simulates it like incandescent or tungsten light, light waves in the red region predominate.  The gem is displaying changes in the light, not changing itself.

Sources

Today, fine Alexandrite is most often found in period jewellery since newly-mined gems are extremely rare.  You'll see fine gems offered at auction with impressive estimates.

The original source in Russia's Ural Mountains has long since closed after producing for only a few decades and only a few stones can be found on the market today.  Material with a certificate of Russian origin is still particularly valued by the trade.

Some Alexandrite is found in Sri Lanka and Zimbabwe and Brazil but very little shows a dramatic colour change.  For many years, Alexandrite was almost impossible to find because there was so little available.

Then in 1987, a new find of Alexandrite was made in Brazil at a locality called Hematita.  The Hematita Alexandrite shows a striking and attractive colours change from raspberry red to bluish green.  Although Alexandrite remains extremely rare and expensive, the production of a limited amount of new material means this wonderful gem is again available.

In Alexandrite the more dramatic and complete the shift from red to green the more rare and valuable the stone.  The other important value factors are the attractiveness and intensity of the two colours, the clarity, and the cutting quality.  Due to rarity, large sizes command extremely very high premiums.