Why Alexandrite Changes Colour in Different Light

One of the most fascinating gemstones in the world has to be alexandrite — and for good reason. Its famous colour change effect makes it feel like magic in mineral form. By day, it might appear green or bluish-green. By night — under incandescent light — it shifts to reddish-purple or even a raspberry hue. It's often described as “emerald by day, ruby by night,” and honestly, that’s not an exaggeration.

But what causes this jaw-dropping transformation?

The Science Behind the Shift

Alexandrite is a variety of chrysoberyl, and its remarkable colour change comes from a combination of chromium content and the way its crystal structure absorbs and reflects light. Natural daylight is rich in blue and green wavelengths, while incandescent (artificial) light leans into red and yellow tones. Alexandrite reacts to those changes with a dramatic shift in colour — a phenomenon known as the alexandrite effect.

This isn’t just some surface-level sparkle; it’s a deep optical response rooted in physics and chemistry. The chromium ions absorb specific wavelengths of light depending on the source, which causes your eyes to perceive different colours.

Inclusions: What Lies Within

During microscopic examination of an alexandrite gemstone, several classic internal features may be visible — and in our case, they were. Here’s what we saw:

• Three-phase inclusions: These are little pockets within the crystal that contain a solid, a liquid, and a gas. They’re often seen in gem-quality stones and can help confirm natural origin.

• Fingerprint inclusions: Named for their resemblance to human fingerprints, these are clusters of tiny fluid-filled fractures.

• Needle-like inclusions: Long, thin inclusions that look like fine threads. In alexandrite, these can sometimes reflect light in interesting ways, adding to the gem’s inner character.

While some inclusions may reduce clarity, they’re not necessarily bad. In fact, they can be a helpful sign that the stone is natural and untreated.

Alexandrite is more than just rare — it’s a gemstone with personality. Every shift in light tells a new story, and its internal inclusions offer a fascinating glimpse into its formation. Whether you’re a gem enthusiast, collector, or just love a good mineral mystery, alexandrite is one of those stones that never gets old.

Next time you see one, tilt it under different lights and watch it come alive — nature’s own optical illusion.