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Hauyne

Last updated: April 2026

Hauyne (also spelled Haüyne) is a rare blue feldspathoid of the sodalite group, found primarily in volcanic phonolite rocks. Gem-quality faceted hauyne is exceedingly rare and sought by collectors; most notable material comes from the Eifel volcanic region of Germany and from Vesuvius, Italy. Its vivid cornflower to royal blue color and extremely low RI (1.494–1.509) and SG (2.44–2.50) are the primary diagnostic characteristics — the RI is so low it falls below the range of a standard refractometer's scale, making it immediately distinguishable from all common blue gem simulants (sapphire 1.762–1.770, blue tourmaline 1.624–1.644, blue topaz 1.609–1.643).

Hauyne has a specific gravity of 2.47 and a refractive index of 1.494–1.509, with a Mohs hardness of 5.5–6.0. It is singly refractive.

Physical & Optical Properties

RI Range1.494–1.509
SG Range2.44–2.50
SG Typical2.47
Hardness (Mohs)5.5–6
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.019
Fluorescence LWStrong orange-pink
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsBlue Violet
SpeciesHauyne (Na,Ca)₄₋₈Al₆Si₆O₂₄(SO₄,S)₁₋₂
Blue Violet

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

Hauyne varies 1.494 – 1.509 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°, watching the boundary. SEEOne boundary that does not move as you rotate. Where it sitsis not fixed for this species, though: published at 1.494 –1.509, and the stone drawn here reads 1.502, between thedashed marks. MEANSSingly refractive — a stationary single edge through thewhole rotation says so. The value itself is only anarrowing: this species spans 1 printed divisions, so areading places your stone inside the range rather thannaming 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

Hauyne Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Hauyne stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. 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

Hauyne SG 2.44–2.50 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 5 other species in the table overlap this range, so a reading here narrows the field to 6 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

Hauyne One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Hauyne 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. 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

Measurement Guides

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

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

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