Phenakite
Last updated: April 2026
Beryllium silicate with unexpectedly high RI for its light weight. Occasionally sold as a colorless diamond simulant or collector stone. Separation from quartz and beryl requires refractometer.
Phenakite has a specific gravity of 2.96 and a refractive index of 1.654–1.670, with a Mohs hardness of 7.5–8.0. It is uniaxial +.
Physical & Optical Properties
RI Range1.654–1.670
SG Range2.95–2.97
SG Typical2.96
Hardness (Mohs)7.5–8
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (+)
Birefringence0.016
Dispersion0.015
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterNot Applicable
PleochroismNone
ColorsColorless
SpeciesPhenakite
Key Differentiators
- Higher RI than quartz (1.544) and higher than white topaz (1.619–1.627) — phenakite RI 1.654–1.670 exceeds topaz
- Lower SG (2.96) than topaz (3.53) — surprisingly light; SG is above beryl's upper range (2.67–2.90)
- Uniaxial positive — biaxial topaz (DR_B+) is easily excluded on polariscope
- Virtually no fluorescence — eliminates danburite (strong blue LW)
Common Simulants
- Diamond: Diamond: extreme hardness (10), thermal probe, RI 2.42, SG 3.52; phenakite SG only 2.97–3.00.
Treatments
- Irradiation (yellow-brown color)
- Fracture Filling (occasional)
Measurement Guides
Identifying a phenakite? GemID walks through these tests in order — RI, SG, fluorescence, and more.
Try GemID Free →Studying for the GIA, FGA, or FEEG practical? Phenakite is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →