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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.

Through the polariscope

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

Apatite SG 3.17–3.23 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 11 other species in the table overlap this range, so a reading here narrows the field to 12 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

Apatite Two colours (dichroic) BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Apatite shows colorless in one window and 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.

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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