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Cinnabar

Last updated: April 2026

A toxic mercury sulfide mineral with a bright scarlet to brick-red color, rarely used as a gem.

Cinnabar has a specific gravity of 8.10 and a refractive index of 2.905–3.256, with a Mohs hardness of 2.0–2.5. It is uniaxial +.

Physical & Optical Properties

RI Range2.905–3.256
SG Range8.00–8.20
SG Typical8.10
Hardness (Mohs)2–2.5
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (+)
Birefringence0.351
Dispersion0.400
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Dichroic
ColorsRed Pink, Brown
SpeciesCinnabar
Red PinkBrown

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

Cinnabar no reading — over the limit 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORead the scale from bottom to top, then look for theliquid’s own faint line near the top. SEEThe field stays dark, with one faint line near 1.81 thatdoes not move when you change stones. MEANSOver the limit — this stone’s index is above the contactliquid’s own, so the liquid is the last thing the instrumentcan still see. That faint line is your proof the instrumentis working rather than dead. It is a RESULT: it eliminatesevery species below 1.81 in one observation. A field darkwith NO line means something else has gone wrong. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Key Differentiators

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a cinnabar? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Cinnabar is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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