Natural vs Synthetic Ruby: How to Tell Them Apart
Natural and synthetic ruby are both genuine corundum with identical optical and physical properties. The refractometer, polariscope, and SG balance cannot distinguish them. The separation relies on microscopic examination of inclusions and growth patterns — natural rubies contain mineral inclusions and straight growth lines; synthetics contain gas bubbles, flux veils, or curved striae depending on the production method.
Property Comparison
| Property | Natural Ruby | Synthetic Ruby | Diagnostic? |
|---|---|---|---|
| RI | 1.762 – 1.770 | 1.762 – 1.770 | No |
| SG | 3.97 – 4.05 | 3.97 – 4.05 | No |
| Hardness | 9 | 9 | No |
| Crystal System | Trigonal | Trigonal | No |
| Optic Character | DR U- | DR U- | No |
| Birefringence | 0.008 | 0.008 | No |
| Fluorescence (LW) | Variable (often strong red) | Often very strong red (Verneuil) | Clue |
| Chelsea Filter | Red | Red | No |
| Pleochroism | Strong dichroic | Strong dichroic | No |
| Inclusions (10x-40x) | Mineral crystals, silk, fingerprints, straight growth | Gas bubbles, curved striae, flux veils | Yes |
The Definitive Tests
- Magnification — growth features (10x-40x). This is the primary separation method. Under darkfield illumination: Flame-fusion (Verneuil): curved growth lines (striae) and round gas bubbles. Flux-grown: wispy flux veils or "rain" inclusions and occasionally flux remnant crystals. Hydrothermal: chevron or zigzag growth patterns. Natural ruby shows straight angular growth lines following the hexagonal crystal structure.
- UV fluorescence intensity. Many Verneuil synthetics fluoresce more strongly (and more uniformly) than natural ruby under LW UV, because they typically contain lower iron content. Extremely strong, even red fluorescence is a clue — but not definitive on its own, because some natural Burmese rubies also fluoresce intensely.
- Inclusion type. Natural ruby may contain rutile silk (intersecting needles at 60/120 degrees), mineral crystal inclusions, healed fractures ("fingerprints"), and zoning. Synthetic ruby inclusions are process-specific: gas bubbles (Verneuil), flux veils (flux), seed-plate remnants (hydrothermal). The presence of any natural-type mineral inclusion is strong evidence of natural origin.
- SW UV phosphorescence. Some flux-grown synthetics show persistent phosphorescence after SW UV exposure that natural rubies do not exhibit. Check for lingering glow after the UV source is removed.
Common Mistakes
- Expecting RI or SG to separate them. Natural and synthetic ruby are chemically identical — both are Al2O3 with Cr3+ chromophore. RI, SG, hardness, and optic character are the same for both. Only microscopic evidence of the growth environment (inclusion suite) can separate them.
- Confusing clarity with synthetic origin. A very clean ruby raises suspicion, but some natural rubies (especially from certain deposits) can be quite clean. Conversely, some synthetics can contain inclusions. Always identify the specific inclusion type — not just clarity grade.
- Missing curved striae in Verneuil synthetics. Curved growth lines can be extremely subtle. Use immersion in a liquid (methylene iodide or RI fluid) and fiber-optic light to improve visibility. Rotate the stone to find the best viewing angle.
Frequently Asked Questions
Are natural and synthetic ruby the same material?
Yes. Both are aluminum oxide (corundum, Al2O3) colored by chromium. They share identical RI (1.762-1.770), SG (3.97-4.05), hardness (9), and crystal structure. The difference is origin — natural rubies formed in the earth over millions of years; synthetic rubies are grown in a laboratory.
Can a refractometer tell natural from synthetic ruby?
No. Both read the same RI range (1.762-1.770) with the same birefringence (0.008). Standard gemological properties are identical. Separation requires microscopic examination of inclusions and growth features.
What is the most common type of synthetic ruby?
Flame-fusion (Verneuil process) synthetic ruby is by far the most common and least expensive. It has been in production since 1902. It is identified by curved growth lines (striae) and round gas bubbles visible under magnification.
Identifying a ruby? GemID guides you through the natural vs. synthetic testing protocol step by step.
Open GemID →