Real alexandrite changes color dramatically, shifting from green in daylight to red under incandescent light. This color change is the stone’s signature trait, but it’s not enough on its own to confirm authenticity. Natural alexandrite is one of the rarest gemstones on earth, and the market is full of synthetics, simulants, and outright fakes. Here’s how to evaluate what you’re actually looking at.
The Color Change Test
Genuine alexandrite’s color shift comes from chromium impurities in its crystal structure. These impurities absorb yellow light (centered around 572 nm wavelength) and also block blue wavelengths below about 480 nm. What’s left after this absorption is green and red, which sit on opposite ends of the visible spectrum. In daylight, which is rich in blue-green wavelengths, your eye picks up the green. Under incandescent bulbs or candlelight, which skew toward red wavelengths, you see ruby-red tones instead.
To test this at home, view the stone under a white LED or fluorescent light (these approximate daylight), then switch to an old-fashioned incandescent bulb or a candle. A real alexandrite shifts from bluish-green or teal in daylight to purplish-red or raspberry under warm light. The shift should be obvious, not subtle. Stones that only change slightly, or that shift between colors like blue and purple rather than green and red, are likely simulants such as color-change sapphire, color-change garnet, or synthetic corundum marketed as “alexandrite.”
That said, lab-grown alexandrite also displays this exact color change, because it has the same chemical composition. So passing the color-change test confirms you have alexandrite, but it doesn’t tell you whether it’s natural or synthetic.
What’s Inside the Stone
The most reliable way to distinguish natural from synthetic alexandrite is examining internal inclusions under magnification, ideally 10x or higher. Natural alexandrite typically contains tiny needle-like inclusions (sometimes called “silk”), mineral crystals, and healed fractures that look like fingerprints. These formed over millions of years of geological activity.
Synthetic alexandrite grown by the flux method can be tricky because it develops “feathers” and fingerprint-like patterns that closely resemble those in natural stones. However, flux-grown synthetics often contain distinctive features: wispy veils of residual flux, two-phase inclusions combining flux residue with spherical gas bubbles, and web-like or net-like patterns of thin flux films radiating outward from a central point. These web patterns, especially when they contain birefringent (light-splitting) components, have been observed only in synthetic alexandrite to date.
Synthetics grown by the Czochralski method (also called “pulled” crystals) are easier to spot. They characteristically display curved growth lines, visible under magnification as gently bending striations running through the stone. Natural alexandrite’s growth lines are straight and angular, reflecting the geometry of its crystal structure. A third method, zone melting, produces swirled or undulating growth patterns that also differ from what you’d see in a natural stone.
A perfectly clean stone with no visible inclusions at all is actually a red flag. Natural alexandrite almost always has some internal features. A flawless stone, especially a large one, is far more likely to be lab-grown.
Checking Physical Properties
Alexandrite is a variety of chrysoberyl, and it has specific physical constants that help separate it from simulants (stones that look similar but are chemically different). Natural alexandrite has a refractive index between 1.746 and 1.755, and a specific gravity of 3.73. These numbers matter because common simulants fall outside this range.
Color-change synthetic corundum, one of the most common alexandrite imposters, has a refractive index of about 1.76 to 1.77 and a specific gravity around 4.00. Color-change garnet sits at a specific gravity near 3.84. A jeweler with a refractometer can quickly check these values. If you’re buying loose stones, a specific gravity test (comparing the stone’s weight in air versus in water) is straightforward and doesn’t require expensive equipment.
Hardness is another clue. Alexandrite rates 8.5 on the Mohs scale, harder than most simulants. It won’t scratch easily, and it can scratch topaz (hardness 8). This isn’t a definitive test on its own, but a stone marketed as alexandrite that scratches too easily is not chrysoberyl.
The Dichroscope Test
Alexandrite is trichroic, meaning it shows three different colors depending on the direction light passes through the crystal. When viewed through a dichroscope (a small, inexpensive handheld tool), authentic alexandrite reveals purple, green, and orange. Only two of these colors appear in any single viewing direction, so you need to rotate the stone to see all three.
This test helps rule out simulants. Synthetic corundum, for example, is dichroic (two colors, not three). Color-change garnet is singly refractive and shows no dichroism at all. If you see only two colors no matter how you rotate the stone, or no color separation at all, it’s not alexandrite.
Lab-grown alexandrite will also show trichroism, since it shares the same crystal structure. So like the color-change test, this confirms you have genuine chrysoberyl but doesn’t distinguish natural from synthetic.
Price as a Reality Check
Natural alexandrite is extraordinarily rare. Fine-quality stones with strong color change routinely sell for thousands of dollars per carat, and top specimens from classic Russian sources can reach $10,000 to $70,000 per carat or more depending on size and color-change intensity. Even lower-quality natural alexandrite with modest color change typically costs several hundred dollars per carat.
If someone is offering you a “natural alexandrite” for $50 to $100 per carat, especially in sizes above one carat, it is almost certainly synthetic or a simulant. Lab-grown alexandrite is widely available and affordable, and there is nothing wrong with it if that’s what you want. But natural stones at synthetic prices simply don’t exist in the marketplace.
Getting a Lab Report
For any significant purchase, a gemological lab report is the gold standard. The Gemological Institute of America (GIA) issues Colored Stone Identification and Origin Reports that assess an alexandrite’s weight, measurements, shape, cutting style, and color. The report states whether the stone is natural or lab-grown, identifies any detectable treatments, and can include an opinion on geographic origin when possible.
Other reputable labs include AGL (American Gemological Laboratories), Gübelin, and SSEF. A legitimate seller of natural alexandrite will either provide an existing report or allow you to have the stone independently certified before finalizing a purchase. Resistance to independent testing is one of the clearest warning signs that a stone isn’t what it’s claimed to be.
If you already own an alexandrite and want to verify it, any certified gemologist with standard equipment (refractometer, dichroscope, microscope) can perform initial screening. For a definitive natural-versus-synthetic determination, especially with high-quality flux-grown synthetics that closely mimic natural inclusions, sending the stone to a major lab is worth the investment.