That ruby in grandma's ring might be glass. That diamond might be cubic zirconia. Or they might be exactly what they appear to be. Without testing, you're guessing—and guessing costs money when you're buying or leaves value on the table when you're selling.
Here's how to identify common gemstones and spot the imitations that fool casual observers.
Understanding What You're Looking For
Gemstone imitations fall into three categories:
Simulants look like a gem but have completely different composition. Cubic zirconia simulates diamond. Red glass simulates ruby. These are relatively easy to identify.
Synthetics have identical chemical composition to natural stones but were created in a lab. Synthetic rubies are real rubies—just not mined from the earth. These are harder to detect without equipment.
Treatments enhance natural stones. Heat treatment, filling, and coating improve appearance but affect value. Disclosure is required but doesn't always happen.
Diamond Testing
Diamonds are most commonly confused with cubic zirconia (CZ), moissanite, and white sapphire. Each has tells.
The fog test: Breathe on the stone like you're fogging a mirror. Diamond disperses heat instantly—fog disappears in 1-2 seconds. CZ holds fog for several seconds. Not conclusive alone but useful as one data point.
The newspaper test: Place the loose stone flat on printed text. Diamond refracts light so strongly that you can't read through it. CZ allows you to see letters. Doesn't work with mounted stones.
Sparkle comparison: Diamond produces both white light (brilliance) and rainbow flashes (fire) in moderate amounts. Moissanite shows excessive rainbow fire—almost disco-ball effect. CZ sparkles but looks slightly "flat" to trained eyes.
Weight: CZ weighs about 1.7 times more than diamond of equal size. If you can weigh a loose stone and calculate expected diamond weight for that size, significant excess suggests CZ.
The definitive test: Thermal testers (diamond testers) cost $15-30 and give instant reliable results. Diamond conducts heat extremely well; simulants don't. Worth buying if you deal with diamonds regularly.
Ruby and Sapphire Testing
Ruby and sapphire are both corundum—same mineral, different colors. Testing methods work for both.
Inclusions: Natural rubies and sapphires contain inclusions visible under 10x magnification. Silk (needle-like rutile crystals), fingerprints, and crystals appear in genuine stones. Perfect clarity in a large ruby is suspicious—either it's synthetic or extraordinarily valuable.
Color distribution: Natural color is rarely perfectly even. Look for color zoning—bands or patches of different saturation. Synthetics often show curved growth lines visible under magnification.
Glass detection: Glass rubies often contain bubbles. Round bubbles trapped in red material = glass. Real rubies never have round air bubbles.
Scratch test: Corundum (hardness 9) scratches everything except diamond. If the stone scratches easily, it's not ruby or sapphire. Note: Don't scratch the table (top flat surface) on potentially valuable stones.
Emerald Testing
Emeralds present unique challenges. Almost all natural emeralds contain inclusions (called "jardin" or garden). Eye-clean emeralds are either extremely rare and valuable or synthetic.
Inclusions are expected: Three-phase inclusions (liquid, gas bubble, and crystal in the same pocket) appear only in natural emeralds. This is the most reliable identifier.
Oil treatments: Most emeralds receive oil or resin treatment to improve appearance. This is industry standard and accepted. Heavy treatment reduces value; minor treatment is normal.
Synthetic tells: Hydrothermal synthetic emeralds may show chevron (V-shaped) growth patterns. Flux-grown synthetics contain wispy veils and flux inclusions that look different from natural jardin.
Quick Tests for Any Stone
Temperature: Real gemstones feel cold to the touch and warm slowly. Glass and plastic reach room temperature quickly. Hold a suspect stone to your lip—cold indicates gem, room temperature indicates imitation.
Weight: Most gemstones feel heavy for their size. Glass and plastic feel lighter. Experience teaches this—handle known real stones until the weight becomes intuitive.
Loupe examination: A 10x jeweler's loupe costs under $15 and reveals details invisible to naked eyes. Bubbles, growth patterns, and inclusions become apparent. This single tool transforms your identification ability.
When Testing Reaches Its Limits
Some identifications require laboratory equipment:
- Distinguishing heated from unheated sapphires
- Identifying origin (Burma ruby vs. Thai ruby)
- Detecting subtle treatments
- Separating high-quality synthetics from naturals
For valuable stones where the difference matters financially, send to a gemological laboratory. GIA, AGL, and Gübelin provide definitive identification with certificates that buyers trust.
The Practical Approach
Use home tests to answer basic questions: Is this glass or potentially real? Does this deserve professional testing?
For inherited jewelry, home testing separates obvious costume pieces from items worth appraising. For purchases, basic tests protect against obvious fraud. For significant value decisions—buying expensive stones or insuring jewelry—professional certification provides certainty that home tests cannot match.
