Whether you're identifying a sapphire, checking if a citrine is natural, or determining if a stone is synthetic, standard gemmological testing is your toolkit for the truth. These are the methods gemmologists rely on every day — non-destructive, reliable, and rooted in science. Here’s a breakdown of the most commonly used tests, what each one does, and the tools involved.
1. Visual Inspection (With Loupe or Microscope)
What it is: The starting point for any identification process. A careful look at a stone’s surface and internal characteristics under magnification.
What it tests for:
- Clarity and inclusions
- Surface features (polish marks, chips, abrasions)
- Growth features (natural zoning, curved vs angular growth)
- Crystal habit and external form
- Signs of treatment (e.g. surface residues from fracture filling)
Tools used:
- 10x triplet loupe
- Binocular gemmological microscope (typically 10x–60x magnification)
- Darkfield and Brightfield illumination
- Fibre-optic light (for targeted inspection)
Bonus insight: Microscopy can often identify synthetic or treated stones by spotting features like curved striae (indicative of flame fusion synthesis) or flash effects in filled fractures.
2. Refractive Index (RI)
What it is: A numerical value that measures how light bends (refracts) as it enters the gemstone.
What it tests for:
- Confirms identity of transparent stones
- Distinguishes between similar-looking gems
- Reveals whether the gem is singly or doubly refractive
Tools used:
- Refractometer (with RI fluid, usually 1.79)
- Polariscope (to check if the stone is singly or doubly refractive)
- Light source (monochromatic sodium light for high accuracy)
Bonus insight: RI is one of the most conclusive tests for transparent gems.
3. Specific Gravity (SG)
What it is: The density of a gemstone compared to water.
What it tests for:
- Identifies or confirms a gem species
- Can sometimes detect synthetics (which may have slightly different SG)
- Helps distinguish between visually similar materials
Tools used:
- Hydrostatic balance (using a fine scale and wire setup)
- Heavy liquids (used in older testing methods)
Bonus insight: Aquamarine and blue topaz may look alike, but they have different SG values — aquamarine is typically around 2.68–2.74, while topaz is closer to 3.53.
4. Polariscope Testing
What it is: A tool that helps determine whether a gemstone is singly or doubly refractive and whether it’s crystalline or amorphous.
What it tests for:
- Optical character (SR vs DR)
- Helps narrow down the crystal system
- Can identify strain (in synthetic or glass materials)
Tools used:
- Polariscope (two polarising filters and a light source)
- Conoscope (for interference figure testing)
Bonus insight: Glass and spinel are singly refractive, but quartz, tourmaline, and sapphire are doubly refractive — this is useful for quickly ruling materials in or out.
5. Spectroscope Analysis
What it is: A tool that breaks white light passing through a gem into its spectral colours, allowing you to see absorption lines.
What it tests for:
- Identifies elements responsible for colour (like chromium or iron)
- Can confirm gem identity (e.g. ruby shows strong chromium lines)
- Helps detect synthetic colouring agents
Tools used:
- Handheld or desk-mounted spectroscope
- Diffraction grating or prism-based
Bonus insight: Natural emeralds often show a distinct chromium absorption spectrum, while green glass does not. It’s a fast way to rule out lookalikes.
6. UV Fluorescence (Shortwave and Longwave)
What it is: A test to observe how a gem reacts to ultraviolet light, which can reveal internal structures or treatments.
What it tests for:
- Identifies specific gems (e.g. ruby often fluoresces red)
- Helps detect fillers and resins
- Reveals synthetic indicators
Tools used:
- UV lamp (longwave at 365 nm, shortwave at 254 nm)
- Viewing cabinet for safety and clarity
Bonus insight: Diamonds can often fluoresce blue under UV light. Synthetic diamonds may fluoresce differently — or not at all.
7. Chelsea Colour Filter
What it is: A filter that allows only specific wavelengths of light through (primarily red and green) to assess colour origin.
What it tests for:
- Helps distinguish natural from synthetic stones (particularly emeralds)
- A quick check for cobalt-dyed stones
- A quick way to sort similar looking stones (like topaz and aquamarine)
- Can help screen for dyed or glass-filled materials
Tools used:
- Chelsea filter
- Strong white light source (e.g. daylight-equivalent torch)
Bonus insight: Natural emeralds often show a reddish or pink glow through the Chelsea filter, while most green glass or peridot will stay green.
8. Dichroscope
What it is: A simple handheld tool that separates the two colours seen in doubly refractive gems.
What it tests for:
- Checks pleochroism (two or three directional colours)
- Helps confirm optical character and gem identity
Tools used:
- Calcite dichroscope
- Strong, diffused light
Bonus insight: Tourmaline typically shows strong pleochroism (green and brown/blue-green), while garnet (though DR in structure) appears SR optically and shows no pleochroism.
Advanced Testing (Lab-Only)
These aren't typically in a gemmologist’s toolkit but worth knowing:
- Raman spectroscopy – precise fingerprinting of mineral species
- FTIR (Fourier-transform infrared spectroscopy) – useful for detecting treatments like resin filling
- EDXRF (Energy-dispersive X-ray fluorescence) – identifies chemical composition
- UV-Vis-NIR spectrophotometry – precise colour and absorption data
Final Thoughts
You don’t need every tool on this list to start testing — but knowing what each test tells you helps you piece together the gem’s identity with confidence. Most gemmologists begin with visual inspection, then confirm or rule out options using RI, polariscope, and SG testing, layering on UV or spectroscopy as needed.