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Star Sapphire

Last updated: April 2026

Star sapphire displays a 6-rayed star from three sets of rutile silk inclusions. Blue and grey-blue are the most common colors; black star sapphire (Thailand, Cambodia) is the most affordable. Sources include Sri Lanka and Myanmar. Linde synthetic star sapphires (Verneuil process) are common in vintage jewelry — very sharp centered star and chalky blue LW fluorescence. Heat treatment destroys silk needles, so fine natural star sapphires are sold unheated.

Star Sapphire has a specific gravity of 4.00 and a refractive index of 1.762–1.770, with a Mohs hardness of 9. It is uniaxial −.

Physical & Optical Properties

RI Range1.762–1.770
SG Range3.95–4.03
SG Typical4.00
Hardness (Mohs)9
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (−)
Birefringence0.008
Dispersion0.018
Fluorescence LWVariable
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Dichroic
ColorsBlue Violet, Black, Colorless, Purple, Red Pink, Yellow Orange
SpeciesCorundum
Blue VioletBlackColorlessPurpleRed PinkYellow Orange

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 Sapphire reads 1.762 – 1.770 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°, reading both edges atevery 45°. SEETwo boundaries 0.008 apart — so close that at this size theyread as one thick line. On the instrument they separate onlywith a steady head and a sharp edge. MEANSThat gap IS the birefringence. Take max minus min across thewhole rotation — a single position understates it, so onereading is an incomplete one. 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 Sapphire Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Star Sapphire 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

Star Sapphire SG 3.95–4.03 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 14 other species in the table overlap this range, so a reading here narrows the field to 15 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 Sapphire Two colours (dichroic) BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Star Sapphire shows violet-blue in one window and greenish-blue 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.

Color-Change SapphirePadparadscha SapphirePink SapphireRubySapphireStar RubyWhite SapphireYellow Sapphire

Key Differentiators

Natural vs. Synthetic

Synthetic star sapphire is commercially available (Flame fusion / Linde process (most common synthetic star sapphire)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Star Sapphire.

Common Simulants

Commonly Confused With

Commonly confused with: Spinel, Glass, Synthetic Spinel, Star Diopside.

Treatments

Measurement Guides

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

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

Try Loupewise Free →

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

Practice exams →