Amethyst
Last updated: April 2026
The purple variety of quartz, its color is caused by iron impurities and irradiation. Ametrine is a natural bicolor variety showing distinct zones of both amethyst (purple) and citrine (yellow-orange) within a single crystal — most commercial material is from Bolivia.
Amethyst 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
- Characteristic color zoning
Natural vs. Synthetic
Synthetic amethyst is commercially available (Hydrothermal). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- Microscopy — inclusions: Parallel two-phase inclusions in planar growth zones (sometimes called 'tiger stripe'), arranged in parallel growth planes — characteristic of natural amethyst. Also: irregular Brazil-law twinning, hematite/goethite/rutile inclusions, fingerprint healed fractures. Synthetic: Seed plate visible as straight rectangular line, often marked by irregular void inclusions along the seed boundary. Regular parallel growth lines from seed. Brazil-law twinning present but lacks tiger-stripe inclusions. Generally inclusion-free.
- UV Fluorescence: Usually inert to very weak dull reddish-brown (LWUV). Not diagnostic. Synthetic: Typically inert. UV is not useful for amethyst origin determination.
- RI / SG: RI 1.544/1.553; SG ~2.65. Synthetic: Identical — confirms quartz species but does NOT distinguish natural from synthetic. Rely on microscopy.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Amethyst.
Commonly Confused With
Commonly confused with: Fluorite.
Treatments
- Heat Treatment → Citrine (if yellow/orange)
- Irradiation (to deepen color)
Related Comparisons
Measurement Guides
Identifying an amethyst? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Amethyst 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 →