Field report · 433
Specific gravity and refractive index, for non-gemologists
Two numbers that gemologists use to identify stones. You don't need the equipment, but knowing what they mean changes how you read product specs.
Two physical properties — specific gravity (SG) and refractive index (RI) — show up on lab reports and product specifications. They're the most reliable non-destructive ways to identify a stone's species. You probably don't have the equipment to measure them yourself, but knowing what they describe will help you read certificates with confidence.
Specific gravity (SG)
SG is a stone's density compared to water. Pure water has SG of 1.00. A stone with SG of 3.5 is 3.5 times denser than water.
Reference values for common gem species:
- Opal: 1.98-2.20
- Quartz (all varieties): 2.65
- Beryl (emerald, aquamarine): 2.65-2.85
- Tourmaline: 3.00-3.25
- Topaz: 3.50-3.57
- Spinel: 3.58-3.61
- Sapphire / ruby (corundum): 3.95-4.10
- Diamond: 3.51
- Zircon: 4.30-4.80
Practical use: if a stone's SG doesn't match its claimed species, something's wrong. A stone sold as "aquamarine" with SG of 3.5 isn't aquamarine — it might be blue topaz or synthetic spinel.
Refractive index (RI)
RI measures how much light bends when entering a stone. The higher the RI, the more dramatically light bends, which (combined with cutting) determines the stone's brilliance.
- Opal: 1.37-1.47
- Quartz: 1.54-1.55
- Beryl: 1.57-1.60
- Topaz: 1.61-1.64
- Tourmaline: 1.62-1.65
- Garnet (varies by species): 1.71-1.94
- Spinel: 1.71-1.74
- Corundum: 1.76-1.78
- Zircon: 1.92-2.01
- Diamond: 2.42
Most stones (except cubic minerals like spinel, diamond, and garnet) are "doubly refractive" — they have two RI values, splitting incoming light into two rays. This birefringence is part of how gemologists distinguish species. Pleochroism (a stone looking different colors from different angles) is a visible consequence of high birefringence.
Why both matter together
Many stones have similar SG OR similar RI but rarely both. A quartz and a glass imitation might both look right by eye, but quartz has SG 2.65 and most glass is 2.4-2.6, with different RI. Combined measurements ID a stone reliably.
Where you'll see these numbers
- Lab certificates from GIA, GRS, etc.
- Detailed product specifications on reputable sites
- Reference texts (Webster's Gems, Schumann, etc.)
For most buyers, SG and RI are facts you can verify on a lab report rather than measurements you'll take yourself. But once you know what they mean, you'll catch identity mismatches that would otherwise slide past.