Sapphire
Last updated: April 2026
Sapphire is any color of corundum except red (ruby). Origin affects value and fluorescence patterns; definitive origin determination requires lab analysis (UV-Vis, trace elements). Padparadscha is a rare pinkish-orange to orangey-pink variety. All corundum colors share the same RI/SG; color and inclusions separate varieties.
Sapphire has a specific gravity of 4.00 and a refractive index of 1.762–1.770, with a Mohs hardness of 9. It is uniaxial −.
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: Corundum Varieties
Key Differentiators
- Strong dichroism
- Hardness of 9
- Hexagonal color zoning
- Silk inclusions
- Chelsea filter: variable — blue sapphire typically inert; pink/yellow/padparadscha may show orange-pink
Natural vs. Synthetic
Synthetic sapphire is commercially available (Flame fusion (Verneuil), Flux, Hydrothermal, and others). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- Microscopy — inclusions: Natural sapphire: rutile silk, zircon crystals with halos, growth zoning (angular/hexagonal), fingerprint healing fractures, liquid inclusions, apatite/columbite crystals. Synthetic: Same as ruby entry above. Flame fusion: curved growth striae and gas bubbles (diagnostic). Flux: flux veils and platinum. Hydrothermal: nail-head spicules, seed plate. Czochralski: typically very clean.
- Spectroscope (blue sapphire): Natural blue sapphire: iron absorption lines at 450nm, 460nm, 471nm typically present — confirm with handheld spectroscope. Synthetic: All synthetic methods: iron lines weak or absent (unless intentionally doped). Near-absence of 450/460nm iron lines in blue sapphire is suspicious for synthetic.
- UV Fluorescence (blue sapphire): Natural blue sapphire: typically inert to weak orange-red. Iron quenches fluorescence in most natural material. Synthetic: Synthetic blue sapphire: generally inert to weak — very similar to natural. Not strongly diagnostic for blue sapphire. Flame fusion yellow sapphire often shows strong orange-yellow LW fluorescence.
- Heat treatment note: Most commercial sapphires are heat-treated to improve color and clarity. Unheated stones command premium. Standard gemological tools cannot reliably detect heat treatment — requires lab (FTIR, UV-Vis). Synthetic: N/A
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Sapphire.
Common Simulants
- Blue spinel: Isotropic (SR) — no trichroism; no facet doubling; RI ~1.718; SG ~3.60.
- Tanzanite: Biaxial positive DR; very strong trichroism (blue-violet-burgundy red); SG 3.35; softer (6.5 Mohs); lower RI (1.691–1.700).
- Iolite: Biaxial negative DR; pronounced trichroism (blue-gray-pale yellow); low SG (2.58–2.66); RI 1.54–1.56; much softer (7–7.5 Mohs).
- Blue tourmaline (indicolite): Uniaxial negative DR; dichroic (blue/blue-green); RI 1.624–1.644; SG 3.06.
- Blue Glass: Isotropic; gas bubbles or swirl marks; no dichroism; conchoidal fracture; SG varies.
Commonly Confused With
Commonly confused with: Spinel, Tanzanite, Iolite, Tourmaline, Glass, Chrysoberyl, Indicolite, Synthetic Spinel, Topaz, YAG, Zircon.
Treatments
- Heat Treatment
- Beryllium (Lattice) Diffusion
- Irradiation
- Glass/Resin Fracture Filling
- Surface Diffusion (Ti/Fe — historical)
Related Comparisons
Measurement Guides
Identifying a sapphire? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Sapphire 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 →