Gem Testing Equipment Overview
Introduction to gemmological instruments, testing workflow, and which tool to use for different identification tasks.
Introduction
Gemmological testing relies on a suite of standard instruments that measure optical and
physical properties to identify gem species, detect treatments, and distinguish natural
from synthetic materials. Mastering these tools is fundamental to gem identification and
forms the core practical skills tested in the FGA Foundation examination.
The primary instruments are the refractometer (measures refractive index, range 1.35–1.81),
polariscope (determines optic character: singly or doubly refractive), dichroscope (reveals
pleochroism in coloured anisotropic gems), spectroscope (displays absorption spectra),
UV lamp (records fluorescence under 365 nm and 254 nm), and gemmological microscope
(examines inclusions and growth features at 10×–60×). The specific gravity balance and
Chelsea filter provide supporting physical and optical data. [1]
A systematic approach using multiple instruments provides the most reliable identification;
no single test is definitive. Stones with RI above 1.81 (diamond, high zircon, demantoid)
exceed the refractometer's range and require specific gravity, thermal probe, or spectroscope
as the primary identification route. [2]
The Gemmological Toolkit
A well-equipped gemmological laboratory includes the following core instruments:
Essential Instruments
| Instrument | Primary Function | Difficulty |
|---|---|---|
| Refractometer | Measure refractive index and birefringence | Moderate |
| Polariscope | Determine optic character (SR/DR) | Easy |
| Dichroscope | Observe pleochroism in coloured stones | Easy |
| Spectroscope | View absorption spectrum | Moderate |
| UV Lamp | Observe fluorescence reactions | Easy |
| Microscope | Examine inclusions and features | Moderate |
| Chelsea Filter | Quick screening for emeralds | Easy |
Supporting Equipment
Beyond the core instruments, a complete toolkit includes:
- Specific gravity balance: For density measurement
- Thermal conductivity probe: Diamond screening
- Loupe (10×): Field examination standard [2]
- Tweezers and stone holders: Safe manipulation
- Cleaning supplies: Cloth, brushes, solvents
- Reference materials: Master stones, comparison samples
Instrument Selection Guide
| Question | Primary Instrument | Supporting Tests |
|---|---|---|
| What is the RI? | Refractometer | SG measurement |
| Is it isotropic or anisotropic? | Polariscope | Refractometer (two readings?) |
| What causes the colour? | Spectroscope | Dichroscope, Chelsea filter |
| Is it natural or synthetic? | Microscope | UV lamp, spectroscope |
| Is it treated? | Microscope | UV lamp, spectroscope |
| What is the origin? | Microscope | Spectroscope, chemistry |
Testing Workflow
High-RI Stones
Recording and Documentation
Accurate documentation is required for professional practice:
What to Record
For each stone examined, document:
- Date and reference number
- Description: Weight, dimensions, cut style, colour
- All test results: RI, optic character, pleochroism, spectrum, fluorescence
- Inclusions observed: With photomicrographs if possible
- Conclusion: Identification with confidence level
Professional Standards
Follow these documentation principles:
- Record observations objectively, without interpretation bias
- Note any limitations or uncertainties
- Keep equipment calibration records
- Maintain chain of custody for valuable items
- Store records securely with appropriate confidentiality
References
- ↑ 1. Read, P. (2014). Gemmology (3 ed.). Butterworth-Heinemann. ISBN: 978-0-08-050722-4. DOI: 10.4324/9780080507224.
- ↑ 2. Anderson, B. (1990). Gem Testing (10 ed.). Butterworth-Heinemann.