Amethyst vs Scapolite: How to Tell Them Apart
Last updated: August 2026
Violet scapolite is one of the sneakiest amethyst look-alikes in the case — and a separation FGA candidates report meeting in the practical exam. The numbers show why it earns its exam slot: scapolite's RI range (1.540-1.572) swallows amethyst's (1.544-1.553) whole, and the SG ranges overlap too. The constants alone cannot finish this identification. The optic sign and the UV lamp can.
Property Comparison
| Property | Amethyst (Quartz) | Scapolite | Diagnostic? |
|---|---|---|---|
| RI | 1.544 – 1.553 | 1.540 – 1.572 | No |
| SG | 2.63 – 2.65 | 2.50 – 2.74 | No |
| Hardness | 7 | 6 – 6.5 | No |
| Crystal System | Trigonal | Tetragonal | No |
| Optic Character | DR U+ (uniaxial positive) | DR U− (uniaxial negative) | Yes |
| Birefringence | 0.009 | 0.020 | Yes |
| Facet Doubling | None visible | Moderate | Yes |
| Fluorescence (LW) | Inert | Strong orange | Yes |
| Fluorescence (SW) | Inert | Variable | No |
| Pleochroism | Weak (purplish-red / bluish-purple) | Strong dichroic (colorless / violet) | Yes |
The Definitive Tests
- LW UV — the orange giveaway. Amethyst is inert under both long-wave and short-wave UV. Scapolite fluoresces strong orange under LW — per the reference data, a response shared by very few gem species and diagnostically useful. An orange glow on an “amethyst” is a loud alarm.
- Polariscope with conoscope — optic sign. Both stones are uniaxial, but the sign differs: quartz (amethyst) is uniaxial positive, scapolite is uniaxial negative. Per the reference data, the optic sign separates them. Quartz adds its own tell — the distinctive “bull's eye” optic figure, specific to quartz.
- Loupe and dichroscope — doubling and dichroism. Scapolite's birefringence of 0.020 produces moderate facet doubling; amethyst's 0.009 shows none under a 10× loupe. The dichroscope reinforces it: scapolite is strongly dichroic (colorless / violet) while amethyst's pleochroism is weak. Chatoyant material tips the scale further — cat's-eye scapolite is a known variety.
Common Mistakes
- Calling it on RI alone. A refractometer reading of 1.545 sits comfortably inside both ranges — scapolite's 1.540-1.572 fully contains amethyst's 1.544-1.553. The same is true of an SG near 2.64. This pair is on exam tables precisely because the headline constants refuse to separate.
- Skipping the UV lamp on violet stones. The single fastest discriminator here costs five seconds: amethyst stays dark, scapolite lights up strong orange under LW. Candidates who treat the UV lamp as an afterthought lose the easiest marks this stone offers.
- Confirming “uniaxial” and stopping. Both stones give a uniaxial interference figure. The separation lives in the sign — positive for quartz, negative for scapolite — and in quartz's bull's-eye figure. Determine the sign; don't just note the figure exists.
Frequently Asked Questions
How do you tell amethyst from violet scapolite?
The measured constants overlap — scapolite's RI (1.540-1.572) contains amethyst's (1.544-1.553), and the SG ranges overlap too. The separation comes from the optic sign (quartz is uniaxial positive, scapolite uniaxial negative) and UV fluorescence: amethyst is inert while scapolite fluoresces strong orange under long-wave UV.
Does amethyst fluoresce under UV light?
No — amethyst is inert under both long-wave and short-wave UV. That makes fluorescence a powerful check against look-alikes: violet scapolite shows a strong orange response under LW that very few gem species share.
Why does this pair appear in gemology practical exams?
Because it punishes shortcut identification. The refractometer and SG readings are consistent with both stones, so candidates must go further — determining the optic sign, checking the bull's-eye figure, using the UV lamp, and noting scapolite's stronger doubling and dichroism.
Identifying a violet stone? GemID walks you through UV, polariscope, and refractometer tests step by step.
Open GemID →Preparing for the FGA practical? Candidates report meeting this exact pair on exam day. GemID’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.
Practice exams →