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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.

Through the polariscope

Cinnabar Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Cinnabar blinks light-dark four times in a full turn. That is doubly refractive. It does NOT tell you uniaxial from biaxial — that is the optic figure, which needs a conoscope. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Cinnabar SG 8.00–8.20 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SCALE ENDS AT 5.0 — THIS BAND RUNS PAST IT Nothing else in the reference table shares this range. A careful hydrostatic reading here names Cinnabar on its own. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Cinnabar Two colours (dichroic) BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Cinnabar shows red in one window and brownish red in the other. Rotate the instrument 45° and the two windows become IDENTICAL — the null. So "both windows the same" means either the stone is singly refractive or you are holding the instrument at the wrong angle, and the only way to tell is to turn it. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

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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