COMPARE GUIDE
Natural AmethystVSPurple Spinel
Side-by-side comparison of physical properties, fluorescence, and crystal system — to identify by observation.
- ◎ is higher in Purple Spinel, giving a numerical distinguishing point.
- ◎ is higher in Purple Spinel, giving a numerical distinguishing point.
- ◎ is higher in Purple Spinel, 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 Spinel | Verdict |
|---|---|---|---|
| Refractive index (RI) | 1.544–1.553 | 1.712–1.736 | B higher |
| Specific gravity (SG) | 2.64–2.66 | 3.58–3.61 | B higher |
| Mohs hardness | 7.0 | 8.0 | B higher |
| Birefringence | 0.009 | — | — |
| Dispersion | — | — | — |
| Property | A: Natural Amethyst | B: Purple Spinel |
|---|---|---|
| Crystal system | 三方晶系 | 等軸晶系 |
| Pleochroism | 弱（紫・赤紫） | — |
| Chelsea filter | inert | pink |
| UV (long-wave) | inert | weak (red) |
| UV (short-wave) | inert | inert |
◆Amethyst (SiO₂ quartz coloured by Fe³⁺ in tetrahedral coordination, Mohs 7, SG 2.65, RI 1.544–1.553, doubly refractive uniaxial positive trigonal, weak , generally inert under LW-UV) versus purple spinel (MgAl₂O₄, Mohs 8, SG 3.58–3.61, RI 1.712–1.736, singly refractive isotropic cubic, no , weak-to-medium red LW-UV ). The specific-gravity difference (2.65 versus 3.58–3.61) is dramatic and diagnostic on hydrostatic balance. The refractive-index difference (1.544–1.553 versus 1.712–1.736) is unambiguous on the refractometer. The polariscope test is decisive: amethyst shows clear under crossed polars while purple spinel shows the dark isotropic cross. Amethyst (0.013) and lustre (vitreous) differ from spinel (0.020) and lustre (sub-adamantine to vitreous). Amethyst is dramatically more abundant and lower-priced ($10–$200 per carat for premium Uruguay or Zambian material) versus purple spinel ($100–$5,000+ per carat). converts amethyst (SiO₂ quartz Fe³⁺) to citrine (yellow Fe³⁺) at 450–500°C — irreversible; spinel does not show this conversion behaviour.
Distinguishing tools
Tools that decide between Natural Amethyst and Purple Spinel 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.