Authenticity Tools
The Refractometer: Measuring a Gem's Bend of Light
A refractometer measures a gemstone's refractive index, a key physical property used by gemologists to help identify gem species.
What refractive index reveals
Refractive index (RI) measures how much a material bends light passing through it. Because many gem species have characteristic RI ranges, a refractometer reading can help narrow down or confirm a stone's identity.
Limitations
A standard gemological refractometer typically reads RI only up to about 1.81, meaning it cannot directly measure diamond (RI 2.42) or certain other high-RI stones, and it works best on stones with a reasonably flat polished facet.
Part of a toolkit, not a standalone verdict
Refractometer readings are most reliable when combined with other observations such as birefringence, pleochroism, specific gravity and microscopic examination — identification rarely rests on a single instrument.
Buyer checklist
- ✓ Use a flat, clean facet for an accurate reading
- ✓ Remember RI alone rarely confirms identity without other tests
- ✓ Understand standard refractometers cannot read diamond's RI directly
Verification tools & references
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