Alexandrite

Named after Tsar Alexander II, alexandrite is among the rarest phenomena in the gem world. A fine stone appears emerald green in daylight and transforms to a vivid raspberry red under incandescent light — a colour change so complete it looks like two different stones.

Why the colour shifts

Alexandrite is a chromium-bearing variety of chrysoberyl. Chromium absorbs light in both the yellow and blue portions of the spectrum, leaving a narrow balance between red and green. The mineral's exact crystal structure determines which of those two dominates — and under daylight (blue-rich) the green wins, while under incandescent light (red-rich) the red dominates.

The historical origin

The original deposit was in the Ural Mountains, discovered in 1830 on the day Prince Alexander came of age. Russian alexandrite from this source shows the most vivid, near-complete colour change — which is why the term "Russian colour change" became a trade benchmark.