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Star Garnet

Last updated: April 2026

Star garnet is the asteriated variety of almandine, displaying a 4-rayed star from two sets of rutile and/or hematite needle inclusions at right angles — the key distinction from star corundum's 6-rayed star. Idaho star garnets occasionally produce a rare 6-rayed star. Star is visible only under a single directional point light. Sources: Idaho (USA) and Rajasthan (India). No commercial synthetic.

Star Garnet has a specific gravity of 4.05 and a refractive index of 1.770–1.830, with a Mohs hardness of 7.0–7.5. It is singly refractive.

Physical & Optical Properties

RI Range1.770–1.830
SG Range3.93–4.30
SG Typical4.05
Hardness (Mohs)7–7.5
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.024
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsRed Pink, Purple, Brown
SpeciesGarnet
VarietyStar Almandine
Red PinkPurpleBrown

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

Star Garnet varies 1.770 – 1.830 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, watching the boundary. SEEOne boundary that does not move as you rotate. Where it sitsis not fixed for this species, though: published at 1.770 –1.830, and the stone drawn here reads 1.800 — the dashedmark is the one end of that range this scale can show. MEANSSingly refractive — a stationary single edge through thewhole rotation says so. The value itself is only anarrowing: this species spans 6 printed divisions, so areading places your stone inside the range rather thannaming it. The top of that range is past the 1.81 ceiling,so some stones of this species give no reading at all. 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

Star Garnet Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Star Garnet stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. 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

Star Garnet SG 3.93–4.30 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 16 other species in the table overlap this range, so a reading here narrows the field to 17 rather than naming the stone. Combine it with a refractive index to separate what is left. 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

Star Garnet One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Star Garnet shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. 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.

Almandine GarnetColor-Change GarnetDemantoid GarnetHessonite GarnetPyrope GarnetRhodolite GarnetSpessartite GarnetTsavorite Garnet

Key Differentiators

Common Simulants

Commonly Confused With

Commonly confused with: Star Ruby.

Measurement Guides

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

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

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Studying for the GIA, FGA, or FEEG practical? Star Garnet 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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