Yellow Diamond
Last updated: April 2026
Yellow diamonds are Type Ia (Cape series, nitrogen in B-aggregates — most common natural fancy yellow) or the rarer Type Ib (isolated nitrogen, canary yellow, common in HPHT synthetics). Cape series stones show characteristic absorption at 415 nm (N3 center) and strong blue LW fluorescence.
Yellow 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
- Type Ia Cape series: N3 absorption at 415nm produces yellow + strong blue LW fluorescence
- Type Ib canary yellow: isolated nitrogen, rare natural, common in HPHT synthetics
- RI 2.417 — highest of any natural gem
- Hardness 10; passes diamond thermal tester
- Strong blue LW fluorescence in most natural fancy yellows (Cape series)
Natural vs. Synthetic
Synthetic yellow diamond is commercially available (HPHT (High Pressure High Temperature) — nitrogen deliberately incorporated for Type Ib yellow; Sumitomo, New Diamond Technology, De Beers, CVD (Chemical Vapor Deposition) followed by irradiation/annealing to produce yellow color). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- Spectroscopy (UV-Vis): Cape series Type Ia: strong N3 absorption at 415nm; secondary bands at 452nm and 465nm. Strong blue LW fluorescence accompanies N3. Spectrum produced by B-aggregate nitrogen formed over geological time. Synthetic: HPHT Type Ib synthetic: no N3 at 415nm; broad absorption around 480nm from isolated nitrogen atoms (Type Ib). No blue fluorescence — often inert or shows orange/yellow fluorescence under LW.
- DiamondView (growth sector imaging): Natural yellow diamonds show irregular octahedral growth patterns. Cape series stones show strong blue fluorescence uniformly across octahedral sectors. Synthetic: HPHT synthetic shows cuboctahedral growth sectors with characteristic alternating blue ({111}) and orange/inert ({100}) sectors. Sector pattern geometry is the definitive diagnostic.
- UV Fluorescence (LW 365nm): Most natural Cape-series yellow diamonds show strong blue LW fluorescence due to N3 center. Some stones also show orange or inert response. Synthetic: HPHT Type Ib synthetics are typically inert or show weak to moderate orange/yellow LW fluorescence. Absence of blue fluorescence in a yellow diamond is a red flag for synthetic origin.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Yellow Diamond.
Common Simulants
- Yellow Sapphire: DR (uniaxial negative); RI 1.762–1.770 vs diamond 2.417; hardness 9; lower dispersion; pleochroism yellow/colorless; fails diamond thermal tester.
- Citrine (Yellow Quartz): DR; RI 1.544–1.553; SG 2.65; hardness 7; much lower RI and SG; fails diamond thermal tester; conchoidal fracture visible under loupe.
- Golden / Imperial Topaz: DR (biaxial positive); RI 1.629–1.637; SG 3.53; hardness 8; perfect basal cleavage {001}; fails diamond thermal tester.
- Heliodor (Yellow Beryl): DR (uniaxial negative); RI 1.568–1.580; SG 2.72; hardness 7.5–8; much lower RI; fails diamond thermal tester.
- Yellow Cubic Zirconia: SR; RI ~2.15 (lower than diamond); SG 5.60–6.00 (much heavier); fails diamond thermal tester; hardness 8–8.5.
- Yellow Scapolite: DR (uniaxial negative); RI 1.550–1.564; SG 2.57–2.74; hardness 5.5–6; much lower RI and hardness; fails diamond tester.
Treatments
- Irradiation (+ annealing)
- HPHT Treatment (color modification)
- Surface Coating
Measurement Guides
Identifying a yellow diamond? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Yellow 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 →