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Rock Crystal

Last updated: April 2026

The colorless and transparent variety of quartz.

Rock Crystal has a specific gravity of 2.65 and a refractive index of 1.544–1.553, with a Mohs hardness of 7. It is uniaxial +.

Physical & Optical Properties

RI Range1.544–1.553
SG Range2.63–2.65
SG Typical2.65
Hardness (Mohs)7
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (+)
Birefringence0.009
Dispersion0.013
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsColorless
SpeciesQuartz
Colorless

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

Rock Crystal reads 1.544 – 1.553 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, 0.009 apart at their widest separation. 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

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

Rock Crystal SG 2.63–2.65 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 24 other species in the table overlap this range, so a reading here narrows the field to 25 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

Rock Crystal One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Rock Crystal 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.

AgateAmethystAmetrineAventurineBloodstoneCarnelianChalcedonyChrysopraseCitrineHawk's EyeOnyxPrasioliteRose QuartzSmoky QuartzTiger's EyeChrysocolla Chalcedony

Key Differentiators

Natural vs. Synthetic

Synthetic rock crystal is commercially available (Hydrothermal (primarily industrial)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

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

Common Simulants

Commonly Confused With

Commonly confused with: Diamond, Topaz, Glass.

Treatments

Measurement Guides

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

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

Try Loupewise Free →

Studying for the GIA, FGA, or FEEG practical? Rock Crystal is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

Practice exams →