Star Ruby
Last updated: April 2026
Star ruby displays a 6-rayed star from three sets of rutile silk inclusions at 60° intervals (asterism), visible under a single point light above a well-cut cabochon. Synthetic star rubies (Linde/Verneuil process) show a very sharp centered star and are common in vintage jewelry from the 1950s–1970s. Heat treatment dissolves silk needles, so fine natural star rubies are typically unheated.
Star Ruby 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
- 6-rayed star under point light source (asterism)
- Caused by intersecting rutile silk needles at 60°/120°
- Hardness 9 — not scratched by anything except other corundum/diamond
- Strong red LW fluorescence typical
Natural vs. Synthetic
Synthetic star ruby is commercially available (Flame fusion / Linde process (most common synthetic star ruby)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Star Ruby.
Common Simulants
- Star spinel: Isotropic (SR); star has 4 rays vs ruby's 6; different fluorescence; check with refractometer.
- Star garnet: Isotropic SR; 4-rayed star; inert under UV; RI 1.770–1.830 range; no Chelsea filter reaction.
- Glass with foil star: Isotropic; star is a surface reflection from foil backing, not internal asterism; visible as flat reflective layer under magnification.
Commonly Confused With
Commonly confused with: Spinel, Garnet, Glass, Star Diopside, Star Garnet.
Treatments
- Low-Temperature Heat Treatment (< 1200°C)
- High-Temperature Heat Treatment (> 1400°C)
- Lead Glass Fracture Filling
- Surface Coating (clarity or star enhancement)
- Beryllium Diffusion (color alteration — laboratory detection only)
Measurement Guides
Identifying a star ruby? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Star Ruby 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 →