Color-Change Sapphire
Last updated: April 2026
Color-change sapphire is corundum colored by vanadium, shifting from blue or blue-gray in daylight to purple, violet, or red-violet in incandescent light. The color change is typically less dramatic than alexandrite's green-to-red shift. The primary identification challenge is separation from alexandrite — resolved by RI (sapphire higher), SG (sapphire higher), and optic character (sapphire uniaxial negative vs. alexandrite biaxial positive). Sources include Sri Lanka and Tanzania.
Color-Change 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.
Through the polariscope
Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.
Specific gravity, and what it rules out
Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.
Through the dichroscope
Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.
Related: Corundum Varieties
Key Differentiators
- RI 1.762 — higher than alexandrite (1.746) — primary physical separator
- SG 4.00 — heavier than alexandrite (3.73)
- Uniaxial negative — alexandrite is biaxial positive
- Color change due to vanadium (V) — alexandrite due to chromium (Cr)
- Generally lacks alexandrite's dramatic green-to-red change — often blue-to-violet/purple
Natural vs. Synthetic
Synthetic color-change 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 color-change sapphire: rutile silk, fingerprint inclusions, zircon crystals with halos, angular color zoning. Often very clean. Sri Lankan material typically clean with fine silk. Synthetic: Same synthetic indicators as all corundum: curved striae + gas bubbles = Verneuil; flux veils + platinum = flux-grown; nail-head spicules = hydrothermal.
- UV Fluorescence: Natural: variable — often weak to inert LW and SW. Some vanadium-rich material shows weak orange or red LW. Generally subdued. Synthetic: Verneuil synthetic color-change corundum: typically weak to moderate fluorescence. Not strongly diagnostic — microscopy is primary.
- Optic character (polariscope/refractometer): Color-change sapphire: uniaxial negative (DR_U-) on polariscope — single optic axis visible as uniaxial interference figure. Synthetic: All synthetic corundum is also uniaxial negative — this test is not for natural vs. synthetic but is essential for separating color-change sapphire (uniaxial) from alexandrite (biaxial positive).
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Color-Change Sapphire.
Common Simulants
- Alexandrite (color-change chrysoberyl): Alexandrite: biaxial positive (DR, biaxial — vs. color-change sapphire which is uniaxial negative), RI 1.746–1.755, SG 3.73 — lower RI and SG than color-change sapphire. Alexandrite shows a typical green-to-red shift; sapphire typically blue-to-purple. RI separates cleanly.
- Color-change garnet: Color-change garnet: isotropic (SR — no birefringence), RI typically 1.740–1.765, SG 3.65–3.80 depending on variety. Indicated by isotropic behavior on polariscope vs. sapphire's uniaxial DR.
- Color-change diaspore (Zultanite): Diaspore: biaxial positive, RI 1.702–1.752, SG 3.35 — lower SG, biaxial. Color change typically green to pinkish in diaspore, different from sapphire's blue-to-purple.
- Color-change glass / simulant: Glass: isotropic, gas bubbles or swirl marks, no dichroism, SG and RI variable but typically much lower. Easily distinguished.
Commonly Confused With
Commonly confused with: Alexandrite.
Treatments
- Heat Treatment
- Beryllium (Lattice) Diffusion
- Glass/Resin Fracture Filling
Measurement Guides
Identifying a color-change sapphire? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Color-Change 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 →