Alexandrite is a rare color-change variety of chrysoberyl that looks green in daylight and shifts to red under incandescent light.
A jeweler holds a stone under a shop light and it glows red; a moment later, near a window, the same stone turns green. Nothing about it changed except the light hitting it. Alexandrite is a color-change gem, and whether you’re buying one, inheriting one, or just trying to understand what you saw, the properties are surprisingly specific.
Here’s what the gem is, what the terms mean, and where quality actually lives.
What Is Alexandrite Made Of?
Alexandrite is the color-change variety of the mineral chrysoberyl, with the chemical formula BeAl₂O₄ — beryllium aluminum oxide.
The color change comes from chromium impurities. Chromium makes the stone transmit red and green light at nearly equal strength, so whichever light source dominates decides which color your eye sees. GIA describes the classic display as bluish green in daylight shifting to purplish red in incandescent light, and also notes green under fluorescent light turning brownish or purplish red under incandescent light.
The famous phrase for it — “emerald by day, ruby by night” — captures the same idea in five words.
The Alexandrite Effect vs. Pleochroism: What Decides The Color?
The alexandrite effect is a color shift caused by the light source itself, not by anything happening inside the stone.
Pleochroism is a different optical property: the different colors visible when you look through the same crystal from different directions. Both can show up in one stone, and mixing them up is the most common mistake people make when describing alexandrite.
Alexandrite Properties At A Glance
| Property | Value | Why It Matters |
|---|---|---|
| Mineral species | Chrysoberyl | Not a corundum, despite the ruby comparison |
| Chemistry | BeAl₂O₄ | Chromium traces drive the color change |
| Mohs hardness | 8.5 | High enough for regular ring wear |
| Refractive index | 1.746–1.755 | Used to identify the stone in a lab |
| Birefringence | 0.008–0.010 | A measurable optical fingerprint |
| Specific gravity | 3.73 | Distinguishes it from lookalikes |
| Typical clarity | Few inclusions | GIA notes stones tend to be relatively clean |
Is Every Color-Change Stone Worth Buying?
No, and the gap between stones is enormous. Fine alexandrite shows green to bluish green in daylight and red to purplish red under incandescent light, with moderately strong to strong saturation — and GIA is direct that the strongest, most vivid stones carry the value. A green-to-red chrysoberyl with muddy or weak tones is still the same mineral, but it isn’t the same gem.
Two more things worth knowing before you spend money:
- GIA evaluates alexandrite rather than grading it on a fixed scale, so there’s no universal letter or number to shop by.
- A GIA colored stone report can confirm whether a stone is natural or lab-grown, name any detectable treatments, and sometimes identify geographic origin.
That last point matters more than it sounds. GIA specifically notes that its reports can distinguish natural from laboratory-grown material, which is the difference between a rare gem and a stone grown to imitate one. If you’re shopping and want to see how the stone looks when it’s actually set, our guide to alexandrite rings for women worth buying covers real settings and prices.
Wear-wise, the 8.5 hardness supports daily use in rings. Some sources describe alexandrite as brittle, and a chrysoberyl can chip if it takes a hard enough knock — so if you’re pairing a high setting with a stone you care about, treat a sharp impact as the thing to avoid. That’s the same handling advice you’d apply to any brittle stone worn on the hand. For identification and quality standards, GIA’s alexandrite quality factors lay out exactly what separates a top stone from an ordinary one.
FAQs
Why does alexandrite change color?
Chromium impurities in the crystal transmit red and green light at close to equal strength. Under daylight, the green wavelengths dominate and the stone looks bluish green. Under incandescent light, the red wavelengths take over. The gem itself never changes — only the light reaching it does.
Is alexandrite the same as chrysoberyl?
Alexandrite is a variety of chrysoberyl, not a separate mineral. The distinguishing feature is the color change: ordinary chrysoberyl doesn’t shift from green to red the way alexandrite does. Same species, same chemistry, different optical behavior caused by chromium content.
Can a lab-grown alexandrite pass as natural?
Not on a proper report. GIA notes its colored stone reports can identify whether a stone is natural or laboratory-grown material. That distinction drives most of the price gap, so a report is worth requesting on any significant purchase rather than trusting a verbal claim from a seller.
References & Sources
- GIA. “Alexandrite Quality Factors” Defines fine color range, saturation expectations, and evaluation standards.
- GIA. “Natural Alexandrite Stone” Source for species, chemistry, and the daylight-to-incandescent color shift.
- GIA. “Alexandrite Geographic Origin Determination” Covers origin testing and the reporting available on colored stones.