Rock Crystal

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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

Key Differentiators

'Bull's eye' optic figure
RI and SG specific to quartz
Absence of color

Natural vs. Synthetic

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

Microscopy: Natural inclusions: liquid feathers, veils, small mineral inclusions, negative crystals, irregular growth lines. Synthetic: Seed plate as straight rectangular boundary, possibly with breadcrumb inclusions. Highly regular parallel growth lines from seed. Typically very clean — suspicious absence of any natural inclusions.
Note: RI and SG identical for all quartz. Inclusions are the only reliable differentiator. Synthetic: Synthetic rock crystal is mainly industrial — gem-quality fake is uncommon. Being too perfect (no inclusions at all) is itself suspicious for large specimens.

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

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Common Simulants

Diamond: Diamond: extreme hardness (10), SG 3.52, RI 2.42, thermal probe; rock crystal SG only 2.65.
Colorless topaz: Topaz: biaxial positive; perfect basal cleavage; SG 3.53; RI 1.619–1.627.
Glass: Isotropic; gas bubbles or swirl marks; no doubling under loupe; lower hardness.

Commonly Confused With

Commonly confused with: Diamond, Topaz, Glass.

Treatments

Surface Coating (titanium oxide 'Mystic' / metallic coatings)
Irradiation (induced color — creates smoky or citrine effect)

Price Context

Natural — low ($/ct)$1

Natural — high ($/ct)$5

NotePer carat; colorless quartz; abundant; value lies in size, cut quality, and clarity

Synthetic — low ($/ct)$1

Synthetic — high ($/ct)$3

Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.

Measurement Guides

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

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Studying for the GIA, FGA, or FEEG practical? Rock Crystal is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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