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
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
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Quartz Varieties
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.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Rock Crystal.
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 1.609–1.643.
- Glass: Isotropic; gas bubbles or swirl marks; no doubling under loupe; lower hardness.
Treatments
- Surface Coating (titanium oxide 'Mystic' / metallic coatings)
- Irradiation (induced color — creates smoky or citrine effect)
Measurement Guides
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 →