Diamond
Last updated: April 2026
The hardest natural substance, known for its brilliance and fire.
Diamond has a specific gravity of 3.52 and a refractive index of 2.417, with a Mohs hardness of 10. It is singly refractive.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Diamond Varieties
Key Differentiators
- Adamantine luster
- High dispersion (fire)
- Hardness of 10
- Thermal conductivity
Natural vs. Synthetic
Synthetic diamond is commercially available (HPHT (High Pressure High Temperature), CVD (Chemical Vapor Deposition)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- UV Fluorescence + Phosphorescence: Variable LW fluorescence (inert to strong blue most common). Pattern angular or irregular. Phosphorescence rare, very weak, < 2 seconds. Synthetic: HPHT: strong chalky blue/green/yellow/orange LW fluorescence with cruciform or hourglass sectoral pattern. Moderate-to-strong phosphorescence (blue/green/orange) lasting many seconds — diagnostically useful. CVD: weak to moderate orange/red/purplish-red fluorescence, patchy or layered pattern. No phosphorescence or extremely brief.
- Polariscope (strain/ADR): Irregular, mottled, chaotic cross-hatch ADR. Not patterned. Synthetic: HPHT: very low to no strain. CVD: 'tatami mat' or 'parquet floor' cross-hatch strain — organized parallel/wavy bands. Diagnostically useful for CVD.
- Microscopy + magnet: Natural inclusions: mineral crystals (garnet, olivine, diopside), feathers, clouds, needles. No metallic inclusions. Synthetic: HPHT: metallic inclusions (dark, opaque, reflective, sometimes with stress halos) — attracted to strong neodymium magnet. CVD: fine pinpoints, diffuse clouds, sometimes linear graphitic inclusions. Both lack typical natural inclusions.
- Lab equipment note: RI/SG identical for all diamond types. Cannot separate origin with standard tools alone. Check the girdle under 10× magnification for laser inscriptions — lab-grown diamonds are often inscribed with 'LGD', the grower's name, or a lab report number. Synthetic: Definitive confirmation requires specialized instruments: DiamondView (De Beers) — UV photoluminescence imaging reveals growth pattern (cuboctahedral for HPHT, layered for CVD); iD100 (Gemological Institute of America) — portable screening device for colorless melee, flags potential synthetics; D-Screen (HRD Antwerp) — automated screening for colorless and near-colorless stones. FTIR (nitrogen type) and Raman spectroscopy also used in lab settings. Refer all high-value stones to an accredited lab.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Diamond.
Common Simulants
- Moissanite: Doubly refractive — facet doubling clearly visible under 10x loupe; thermal probe may not differentiate; use moissanite tester (electrical conductivity) or loupe test.
- Cubic Zirconia (CZ): Very high SG (5.6–6.0) — stone feels heavy; isotropic; thermal probe reads cold; no natural inclusions.
- White Sapphire: Doubly refractive uniaxial negative; RI 1.762–1.770; SG 4.00; appears slightly "fuzzy" face-up vs diamond brilliance.
- White Topaz: Biaxial positive DR; perfect basal cleavage; RI 1.609–1.643; SG 3.53; lower brilliance.
- White zircon: Strong birefringence — facet doubling visible under loupe; very high SG (4.67–4.73); RI 1.925–1.984.
Commonly Confused With
Commonly confused with: Moissanite, Cubic Zirconia, White Sapphire, White Topaz, White Zircon.
Treatments
- Laser Drilling
- Fracture Filling (glass/resin)
- HPHT Color Enhancement
- Irradiation + Annealing (fancy color)
- Surface Coating
Related Comparisons
Measurement Guides
Identifying a diamond? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Diamond is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →