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Ruby

Last updated: April 2026

Ruby is the red variety of corundum. Origin affects value and fluorescence: Burmese (Mogok) rubies have low iron and show strong red LW fluorescence; Thai/Cambodian rubies have higher iron and weaker fluorescence; Mozambique rubies (Montepuez) show strong fluorescence similar to Burmese. Origin determination requires lab analysis (EDXRF, UV-Vis spectrum).

Ruby 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 Range3.97–4.05
SG Typical4.00
Hardness (Mohs)9
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (−)
Birefringence0.008
Dispersion0.018
Fluorescence LWVariable
Fluorescence SWVariable
Chelsea FilterRed
PleochroismStrong Dichroic
ColorsRed Pink
SpeciesCorundum
Red Pink

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

Ruby 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

Ruby Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Ruby 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

Ruby SG 3.97–4.05 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

Ruby Two colours (dichroic) BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Ruby shows purplish-red in one window and orangey-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.

Color-Change SapphirePadparadscha SapphirePink SapphireSapphireStar RubyStar SapphireWhite SapphireYellow Sapphire

Key Differentiators

Natural vs. Synthetic

Synthetic ruby is commercially available (Flame fusion (Verneuil), Flux (Chatham, Ramaura, Kashan), Hydrothermal, and others). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

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

Common Simulants

Commonly Confused With

Commonly confused with: Spinel, Tourmaline, Garnet, Glass, Almandine Garnet, Pyrope Garnet, Rhodolite Garnet, Rubellite, Synthetic Spinel, YAG.

Treatments

Related Comparisons

Natural Vs Synthetic RubyPink Sapphire Vs RubyRuby Vs GarnetRuby Vs Red Spinel

Measurement Guides

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

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

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Studying for the GIA, FGA, or FEEG practical? Ruby 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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