GEMSTONE FIELD GUIDE
Amethyst
A color variety of quartz, not a separate mineral species.
Amethyst, the purple variety of quartz, NMNH 125254. Museum reference specimen; not NumisTrends inventory.
Amethyst is the purple variety of quartz. Here the natural crystals remain in an uncut mineral cluster.
Photo: Chip Clark / Smithsonian National Museum of Natural History, CC0. · Source ↗ · CC0 1.0 · resized for the webA verified 3D specimen capture is not available for this entry. No generated stand-in is used.
AT A GLANCE
Start with the evidence.
Establish quartz identity before asking whether the amethyst is natural. Clean natural and hydrothermal synthetic material can be difficult to separate.
- GIA — Large primary fluid inclusion in amethyst (Fall 2016) ↗
- GIA — Laboratory growth of gem materials (Summer 2024) ↗
- Refractive index (RI)
- 1.544–1.553
- Specific gravity (SG)
- 2.66 · typical
- Mohs hardness
- 7
typical principal-index interval
How to read this value
Published reference interval, not necessarily the two readings of every specimen and not an exhaustive identity boundary. Composition, orientation, wavelength, instrument and condition matter.
How to read this value
GIA publishes a typical value without a numerical tolerance; range left null.
How to read this value
Scratch resistance, not toughness. This ordinal scale is not linear.
These are reference properties, not measurements of the pictured specimen. Reference images are educational—not NumisTrends sale inventory.
See the details and limitations ↓01 · IDENTIFY
Build a profile, not a guess.
Typical material-level reference; not a measurement, identification, origin, treatment determination, or grading result for any specimen.
Refractive index (RI)1.544–1.553
typical principal-index interval
Published reference interval, not necessarily the two readings of every specimen and not an exhaustive identity boundary. Composition, orientation, wavelength, instrument and condition matter.
Specific gravity (SG)2.66 · typical
GIA publishes a typical value without a numerical tolerance; range left null.
Mohs hardness7
Scratch resistance, not toughness. This ordinal scale is not linear.
Birefringence0.009
Typical crystalline-quartz value; not a measured result for a displayed stone.
Optic characterUniaxial positive
CleavageNot normally evident / no practical cleavage
Gem-A's structural discussion describes quartz cleavage as scarcely developed; ordinary breakage is not a prominent gemstone cleavage like topaz or diamond. Avoid treating flat crystal growth faces as cleavage evidence.
Open a property for its scope and source. RI and specific gravity have no units. Mohs describes scratch resistance—not resistance to breaking. Do not scratch a stone to test it.
What can look similar?
Purple glass
Glass is manufactured in colors that imitate amethyst.
How to investigate. Have the material's optical properties and internal features examined rather than relying on purple color.
No universal glass-versus-amethyst UV rule is supplied here.
Hydrothermal synthetic amethyst
Both natural and synthetic quartz can be sufficiently clean to lack obvious origin clues.
How to investigate. Use trained examination of growth evidence and laboratory follow-up when necessary.
Absence of a visible seed plate is not evidence of natural growth.
02 · TESTS & ULTRAVIOLET
What a test can—and cannot—tell you.
Results depend on the specimen, instrument and method. This is a reference for understanding evidence, not a home authentication or treatment verdict.
UV reaction
- Long-wave UV
- The natural amethyst in GIA's 2016 case study was inert under longwave UV.
- Short-wave UV
- The same specimen was inert under shortwave UV.
This is a documented specimen result, not a verified universal range for natural or synthetic amethyst. No UV-only natural-versus-synthetic decision rule is approved in this record.
Optical properties and microscopy
Whether the specimen's properties and internal features are consistent with amethyst.
Reference observation. GIA's cited natural specimen showed double refraction and a uniaxial-positive optic character.
Limit. Those measurements belong to the documented specimen; they are not proof of natural origin for another stone.
Growth-origin assessment
Evidence of natural or hydrothermal synthetic growth.
Reference observation. Brazil-law twinning and retained seed material can be useful observations.
Limit. Twinned synthetic seeds exist, seed remnants may be cut away, and FTIR results can be inconclusive. Use combined evidence.
Real laboratory examples
Documented cases, not test results of the reference specimens shown on this page.
A documented natural fluid inclusion
GIA examined an amethyst containing a large primary aqueous inclusion with a movable gas bubble; the authors used that inclusion to establish the natural origin of this particular stone.
The takeaway. The inclusion's context matters; this case does not make every bubble-like feature proof of natural origin.
Use a qualified gemologist for identification work. Do not expose eyes or skin to UV sources, or perform scratch, heat, acid or solvent tests on a gemstone. Do not assume that an absent UV reaction rules a material out.
03 · TREATMENTS & CARE
Protect the stone you actually have.
Disclosed treatments, fractures and settings can change which cleaning methods are appropriate.
Avoid heat, steam and abrupt temperature changes. Prolonged intense light can fade some amethyst; GIA recommends warm soapy water for routine cleaning.
04 · SOURCES & SCOPE
Know where each fact comes from.
Original NumisTrends explanations with primary references. This is not a replacement for gemological training, a laboratory report, or the examination of your own stone.
Properties reviewed: 2026-09-07 · Identification notes reviewed: 2026-09-07
- gia.edu · amethyst ↗
- gem-a.com · ↗
- gem-a.com · JoG1950_2_6.pdf ↗
- gia.edu · buyers guide ↗
- gia.edu · amethyst care cleaning ↗
- gia.edu · citrine ↗
NumisTrends is independent of GIA, Gem-A and the museums cited. A matching property cannot by itself establish identity, natural versus laboratory-grown origin, treatment, geographic origin, grade or value.