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.
Physical & Optical Properties
RI Range1.757–1.804
SG Range3.64–3.68
SG Typical3.66
Hardness (Mohs)6–6.5
Crystal SystemHexagonal
Optic CharacterDR Uniaxial (+)
Birefringence0.047
Dispersion0.046
Fluorescence LWInert
Fluorescence SWStrong Blue White
Chelsea FilterNot Applicable
PleochroismModerate
ColorsBlue Violet
SpeciesBenitoite
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? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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