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Apatite

Last updated: April 2026

Neon blue-green apatite from Madagascar is often confused with paraiba tourmaline or aquamarine. Hardness is exactly Mohs 5 — a knife blade or steel file will scratch it. Yellow apatite shows strong yellow-orange LW fluorescence.

Apatite has a specific gravity of 3.20 and a refractive index of 1.628–1.649, with a Mohs hardness of 5. It is uniaxial −.

Physical & Optical Properties

RI Range1.628–1.649
SG Range3.17–3.23
SG Typical3.20
Hardness (Mohs)5
Crystal SystemHexagonal
Optic CharacterDR Uniaxial (−)
Birefringence0.006
Dispersion0.013
Fluorescence LWVariable
Fluorescence SWVariable
Chelsea FilterInert
PleochroismModerate
ColorsBlue Violet, Green, Yellow Orange, Colorless
SpeciesApatite
Blue VioletGreenYellow OrangeColorless

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

Apatite varies 1.628 – 1.649 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 0.006 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. The pair drawn here isone specimen: this species is published at 1.628 – 1.649,and the two edges slide together anywhere across it, betweenthe dashed marks. MEANSThat gap IS the birefringence — and it is the stable part.Where the pair sits varies by 2 printed divisions from stoneto stone, so the gap identifies and the position onlynarrows. Take max minus min across the whole rotation; asingle position understates it. 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

Related Comparisons

Paraiba Tourmaline Vs Apatite

Measurement Guides

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

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

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