Chrysoberyl (var. Alexandrite)

Alexandrite is among the most renowned colour-changing minerals, recognized as a green, chromium-bearing variety of chrysoberyl. Though not a distinct species. It was first discovered in Russia in the 1800s and named after Tsar Alexander II.  Alexandrite crystals often form in a "sixling" (or more accurately, "trilling") twin habit, though it's unclear if this specimen exhibits twinning. With strong backlighting, the crystal shows an impressive colour change: transitioning from a rich purplish-red in warm light to a vibrant green in cool light. Admittedly it was tricky to see it with this dark crystal but not impossible, the colour change can be seen along the top edge where the crystal is the thinnest, and with a powerful flashlight that can offer different spectrums. Very hard to see the colour change just by holding it up to the light!

The effective substition of Fe³⁺ on the mirror-symmetry site has a strong effect to the Cr³⁺ lifetime, Cr being responsible for the stunning colour effect. While the crystal appears dark, backlighting reveals internal gemminess. Fine specimens are rare, with only a few localities worldwide this classic material from Brazil is increasingly hard to find on the market.