COMPARE GUIDE
Natural AmethystVSPurple Glass
Side-by-side comparison of physical properties, fluorescence, and crystal system — to identify by observation.
- ◎ is higher in Natural Amethyst, giving a numerical distinguishing point.
Note: properties whose numeric ranges do not overlap are auto-extracted. Both are measurable with a refractometer or heavy-liquid bath.
| Property | A: Natural Amethyst | B: Purple Glass | Verdict |
|---|---|---|---|
| Refractive index (RI) | 1.544–1.553 | 1.480–1.700 | Overlap |
| Specific gravity (SG) | 2.64–2.66 | 2.30–4.50 | Overlap |
| Mohs hardness | 7.0 | 5.0–6.0 | A higher |
| Birefringence | 0.009 | — | — |
| Dispersion | — | — | — |
| Property | A: Natural Amethyst | B: Purple Glass |
|---|---|---|
| Crystal system | 三方晶系 | 非晶質 |
| Pleochroism | 弱（紫・赤紫） | — |
| Chelsea filter | inert | — |
| UV (long-wave) | inert | — |
| UV (short-wave) | inert | inert |
◆Natural amethyst (SiO₂ quartz colored by Fe⁴⁺-substituted iron, Mohs 7, SG 2.65, RI 1.544–1.553, doubly refractive, 0.013) is the natural species that purple glass most often imitates. Refractometer reads 1.544–1.553 for amethyst versus 1.50–1.70 for glass (overlap requires combined testing). The decisive separation comes from polariscope examination (amethyst doubly refractive, glass singly refractive) and from examination at 10× (amethyst shows tiger-stripe color zoning, Brazil-law twinning, and characteristic angular growth patterns; glass shows regular round gas bubbles and pressing-die seams). Mohs 7 amethyst is unaffected by a steel-knife scratch test that easily marks Mohs 5–6 glass. The Brazilian (Minas Gerais), Uruguayan, Zambian, and Russian (Ural Mountains) deposits supply the bulk of commercial amethyst that purple glass imitates. Disclosure as 'glass' or 'imitation amethyst' is required under FTC Jewelry Guides §23.25.
Distinguishing tools
Tools that decide between Natural Amethyst and Purple Glass in practice.
Note: this page is auto-generated from published GIA / CGL reference values. Individual stones, treatments, and synthetics may fall outside these ranges. For a definitive identification, consult an accredited gem laboratory.