Synthetic Spinel
Last updated: August 2026
Flame-fusion synthetic spinel was the dominant birthstone simulant from the 1930s–1980s. Estate jewelry from this era frequently contains it as a simulant for sapphire, aquamarine, ruby, and alexandrite. The cobalt-blue variety shows a strong red Chelsea filter reaction. All varieties show anomalous double refraction ('tabby extinction') on the polariscope. RI is slightly elevated vs. natural spinel due to excess alumina in the Verneuil process.
Synthetic Spinel has a specific gravity of 3.63 and a refractive index of 1.725–1.728, with a Mohs hardness of 8. 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.
Key Differentiators
- Strong 'tabby extinction' (checkerboard-like ADR) on polariscope — diagnostic for synthetic origin
- Cobalt-blue variety: vivid red through Chelsea filter — immediately diagnostic
- RI 1.725–1.728 — slightly elevated vs natural spinel (1.712–1.730)
- Dominant simulant in 1940s–1980s estate jewelry
- Unusually clean under loupe — lack of natural inclusions
Natural vs. Synthetic
Synthetic Spinel is commercially available (Flame fusion (Verneuil)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- Polariscope: Natural spinel: may show mild ADR but typically more uniform extinction Synthetic: Synthetic: very strong tabby (checkerboard-like) anomalous extinction — diagnostically useful
- Chelsea Filter (cobalt-blue varieties): Natural blue spinel: varies; may show weak red response Synthetic: Synthetic cobalt-blue: vivid strong red — conclusive indicator of cobalt coloring
- Refractometer: Natural spinel: typically 1.712–1.730 Synthetic: Synthetic flame-fusion: 1.725–1.728 (elevated due to excess alumina)
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Synthetic Spinel.
Commonly Confused With
Commonly confused with: Sapphire, Ruby, Aquamarine, Emerald, Peridot, Alexandrite.
Related Comparisons
Measurement Guides
Identifying a synthetic spinel? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Synthetic Spinel 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 →