How to identify real Alexandrites vs lab-grown ones: Expert identification guide

Alexandrite is one of the rarest gemstones you can buy. It is also one of the most copied. Both natural and lab-grown alexandrite are real chrysoberyl and can look similar. The differences show up in the way the stone changes color, the internal growth features, and how clean and bright it looks for the price. This guide explains what to look for, why it matters, and how to test a stone from home and with simple tools.

What makes alexandrite unique

Alexandrite is chrysoberyl colored by chromium. Chromium absorbs parts of the light spectrum. In daylight (which is rich in blue/green), alexandrite looks greenish. In warm incandescent light (rich in red), it looks red to purple. This dramatic change is the “alexandrite effect.”

That same chromium creates strong pleochroism (different colors from different directions). It also produces a diagnostic absorption pattern in a handheld spectroscope. These features confirm the species, but they do not prove if it is natural or lab-grown. For that, you need to study growth features and the character of the color change.

Test the color change the right way

Color-change results depend on the light you use. Poor lighting will fool you, so control it.

• Daylight test: Use bright, diffused daylight. A north-facing window or open shade works. Avoid fluorescent office lights—they have spiky spectra.
• Incandescent test: Use a warm incandescent or halogen bulb (2700–3000K). Candlelight also works.
• Observe the change: Note the day color (usually bluish-green to yellowish-green) and the incandescent color (purplish-red to brownish-red).
• Estimate coverage: What percentage of the face changes? Natural stones commonly show a partial shift (40–80%). Lab-grown stones often swing closer to 90–100% with very pure hues.

Why this helps: Lab production can tune chromium and reduce iron, pushing purer reds and greens and a bigger, cleaner switch. Natural stones often carry trace elements that mute the red (adding brown) or the green (adding yellow). A flawless, dramatic switch in a large, cheap stone is a warning sign.

Judge the hues, not just the change

Collectors pay most for a strong shift, but the quality of the hues matters more than the percentage.

• Natural trend: Day color is slightly bluish-green or mossy green. Night color leans to purplish-red or raspberry, often with a faint brown or gray mask.
• Typical lab-grown: Day color is vivid emerald-like green or teal. Night color is vivid magenta or grape-purple. The colors look “clean,” almost too perfect.

Why this helps: Real alexandrite seldom hits pure traffic-light green to ruby red without some modifier. If it does—and the stone is large and spotless—suspect a lab-grown stone.

Look for pleochroism (with or without tools)

• By eye: Rotate the stone. Natural alexandrite shows different body colors from different angles—greenish one way, orangey or reddish another. The effect is stronger in daylight.
• Dichroscope: Expect two strong colors. In alexandrite, these often correspond to green and red/orange. Pleochroism in color-change garnet or spinel is weak to absent.

Why this helps: Chrysoberyl is doubly refractive and strongly pleochroic. Single-refractive imitators (garnet, spinel, glass) won’t show this behavior.

Use magnification to read growth features

A 10x loupe or microscope is the most practical way to separate natural from lab-grown. You are not looking for dirt. You are reading the crystal’s growth history.

Natural alexandrite often shows:

• Healed “fingerprints”: Wispy, branch-like patterns where cracks healed. Common in natural gems.
• Straight growth zoning: Fine, straight or angular lines. Color zoning may be uneven.
• Twin lamellae (Brazil-law twinning): Thin parallel lines or step-like structures, sometimes in sets (trillings).
• Negative crystals and pinpoints: Angular voids or tiny crystals with sharp edges, sometimes in trails.
• General look: Busy but subtle. Inclusions are not “engineered.”

Lab-grown alexandrite commonly shows:

• Hydrothermal growth: Chevron or herringbone zoning. Nail-head spicules (tubes with thicker ends) aligned to the seed. Wispy veil-like channels.
• Flux growth: Reflective metallic-looking platelets, brownish “flux fingerprints,” droplets and wispy flux residues clustered in planes.
• Pulled (Czochralski) or flame-fusion: Faint curved growth lines and gas bubbles. Curved striae are synthetic red flags in any gemstone.
• General look: Patterns repeat and look “manufactured.” Inclusions can be diagnostic if you know what they are.

Why this helps: Growth method leaves a signature. Nature makes straight or twinned zoning and healed features. Labs create chevrons, curved lines, or flux residues that nature does not.

Quick instrument checks

• Refractive index (RI): Alexandrite (chrysoberyl) is about 1.746–1.755 with birefringence ~0.008–0.010. Natural and lab-grown read the same. RI confirms species, not origin.
• Specific gravity (SG): ~3.73. Again, species check only.
• Spectroscope: Look for chromium absorption, including sharp lines in the red and a broad band in the yellow-green. This verifies alexandrite-type coloring but cannot separate natural from lab-grown.
• UV fluorescence: Many naturals are inert to weak red. Some lab-grown stones glow stronger red under long-wave UV due to higher chromium and fewer quenchers. Use this as a clue, not proof.
• Polariscope: Doubly refractive, biaxial. Garnet and spinel are singly refractive; glass is amorphous.

Why this helps: These tests confirm what the stone is and rule out common look-alikes. Only inclusion study and growth patterns reliably sort natural from lab-grown without advanced lab analysis.

Size, clarity, and price clues

• Rarity: Fine natural alexandrite above 1 ct is very rare. Clean stones above 2–3 ct are exceptional.
• Clarity: Naturals often have small inclusions. Lab-grown stones are commonly eye-clean or flawless.
• Price: A large, clean, vivid, hard-switching stone at a bargain price is almost always lab-grown or a different gem.

Why this helps: Rarity sets the price. If the quality looks too high for the money, something else is going on.

Common look-alikes and quick tells

• Color-change garnet: Green to red, but singly refractive and usually no pleochroism. Often shows mixed hues (teal to wine) and higher dispersion “sparkle.”
• Color-change sapphire: Blue to purple. Uniaxial, different RI (about 1.76–1.77), weaker pleochroism in these colors.
• Color-change spinel: Singly refractive, no pleochroism. Often cleaner and cheaper in larger sizes.
• Glass/assembled stones: Obvious round bubbles, very low RI, no pleochroism, soft surface wear.

Why this helps: Pleochroism plus RI quickly separates chrysoberyl from most mimics.

Red flags in listings

• Vague wording: “Lab alexandrite” can mean lab-grown chrysoberyl, but sellers sometimes misuse it for color-change sapphire or glass.
• Over-the-top photos: Hyper-saturated green and magenta in every frame suggest filters or LEDs, not honest lighting.
• No daylight/incandescent photos: You should see both.
• Big size, perfect clarity, low price: Expect lab-grown.

When to get a lab report

If the stone is valuable, get a report from a reputable gem lab. A report can:

• Confirm species: natural chrysoberyl (alexandrite) or synthetic alexandrite.
• Describe color-change and any treatments.
• Sometimes suggest origin based on inclusions and chemistry, though origin is not always possible.

Why this helps: Some modern synthetics are very convincing. Only lab tools like advanced spectroscopy and trace-element analysis settle hard cases.

Step-by-step identification checklist

• View in proper daylight and incandescent light. Note both hues and the percentage of the change.
• Check for pleochroism by rotating the gem or using a dichroscope.
• Use a loupe to inspect inclusions:
  • Natural: straight zoning, fingerprints, twin lamellae, angular negative crystals.
  • Lab: chevron zoning, nail-head spicules (hydrothermal), flux platelets/veils, curved striae or bubbles (pulled/flame-fusion).
• Measure RI and SG to confirm chrysoberyl and rule out look-alikes.
• Use a spectroscope to verify chromium absorption (supports alexandrite color mechanism).
• Note fluorescence under LW UV; strong red can hint at lab growth but isn’t proof.
• Sense-check size, clarity, and price. Rare combinations at low prices are a red flag.
• If still unsure, get a lab report.

Handling tips and common mistakes

• Do not scratch-test: You can ruin the gem. Hardness tests are destructive and unnecessary.
• Use clean lighting: LEDs vary widely. If you test under mixed or colored LEDs, your results will be unreliable.
• Photograph in both lights: Take one photo in daylight, one under incandescent. Set white balance to neutral to avoid exaggeration.
• Don’t judge from a single frame: Color-change gems drift with small lighting shifts. Compare several views.

Bottom line

There is no single “magic” test that separates natural and lab-grown alexandrite. You get there by stacking evidence: honest lighting tests, strong pleochroism, natural-looking inclusions, and realistic size/price. Instruments confirm the species and rule out imitators. In tough cases, a lab report is the final word. If you build the habit of checking the why behind each clue—the lighting spectrum, the growth patterns, the chemistry—you will spot most lab-grown stones before you reach the checkout.