Benitoite
Last updated: April 2026
Found almost exclusively in San Benito County, California, in association with neptunite and natrolite. Extremely rare in faceted form. Vivid blue-white SW UV fluorescence is the primary field diagnostic.
Benitoite has a specific gravity of 3.66 and a refractive index of 1.757–1.804, with a Mohs hardness of 6.0–6.5. 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.
Key Differentiators
- Vivid strong blue-white SW UV fluorescence — near-unique diagnostic property
- Very high dispersion (0.046) — produces intense fire similar to diamond
- Uniaxial positive — separates from blue sapphire (uniaxial negative)
- RI near refractometer limit (1.757–1.804); second reading may be OTL
- California state gem — provenance almost exclusively San Benito County, CA
Measurement Guides
Identifying a benitoite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Benitoite 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 →