Loupewise / Reference / Pearl
Properties Refractometer Key Tests Treatments Pearl has a specific gravity of 2.74 and a refractive index of 1.520–1.690, with a Mohs hardness of 2.5–4.5. It is aggregate.
Physical & Optical Properties
RI Range 1.520–1.690
SG Range 2.60–2.85
SG Typical 2.74
Hardness (Mohs) 2.5–4.5
Crystal System Aggregate
Optic Character Aggregate
Fluorescence LW Variable
Fluorescence SW Inert
Chelsea Filter Inert
Pleochroism None
Colors Colorless, Red Pink, Yellow Orange, Black, Brown, Green, Blue Violet
Species Organic
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
Pearl
varies 1.520 – 1.690
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
REFRACTIVE INDEX
DO Rotate the stone through a full 360°, watching the boundary.
SEE One boundary that does not move as you rotate — and it is often vague rather than crisp. Where it sits is not fixed for this species: published at 1.520 – 1.690, and the stone drawn here reads 1.605, between the dashed marks.
MEANS This is an aggregate — a mass of microscopic crystals — and on this instrument it looks exactly like a singly refractive stone. The refractometer cannot tell the two apart. The polariscope can: an aggregate stays light through a full rotation, where a singly refractive stone stays dark.
Drawn from the refractive index above, on a 1.40–1.81 scale.
How to take this reading .
Through the polariscope
Pearl
Light throughout (AGG)
0°
45°
90°
135°
ROTATION BETWEEN CROSSED POLARS
Pearl stays light in every position. Light crosses many randomly oriented grains, so some
component always reaches the analyser. On a refractometer an aggregate is indistinguishable from
a singly refractive stone; this is the test that separates them.
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
Pearl
SG 2.60–2.85
WHERE IT SITS, AND WHAT ELSE IS THERE
1.0
2.0
3.0
4.0
5.0
SPECIFIC GRAVITY
41 other species in the table overlap this range, so a reading here narrows the field to 42 rather than
naming the stone. This is the crowded part of the scale — the same careful measurement is worth far more
higher up. Pearl is porous: it soaks up water during immersion, which raises the weight in water and
makes the SG read HIGH. Heavy liquids damage porous material, so there is no clean fallback — treat any
figure here as unreliable.
Drawn from the specific-gravity range above, against
every other species in the table.
How to take this reading .
Through the dichroscope
Pearl
One colour (no pleochroism)
IDENTICAL AT EVERY ANGLE
Pearl 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.
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
Characteristic orient/luster Layered structure (nacre) Organic origin
Treatments
Bleaching (whitening/uniformizing) Dyeing (black, pink, etc.) Surface Coating / Lacquering Irradiation (artificial black color — Tahitian imitation)
Identifying a pearl? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
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Studying for the GIA, FGA, or FEEG practical? Pearl 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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