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How to use a refractometer (and the rest of the gemmological toolkit)

Free FGA equipment module, step-by-step

The standard gemmological laboratory uses a small, well-defined set of optical instruments (refractometer, polariscope, dichroscope, spectroscope, microscope, UV lamp, and hydrostatic balance), and competence with each is a Gem-A Foundation examination requirement. The refractometer is the workhorse: a single accurate reading of refractive index plus a birefringence value places the unknown stone in a shortlist of typically three or four species, which the polariscope (optic character) and spectroscope (absorption pattern) then resolve. This hub collects the 13 equipment articles aligned with the FGA Foundation syllabus: overview, refractometer, polariscope, dichroscope, spectroscope, microscope, UV lamp, SG measurement, conoscope, Chelsea filter, diamond screening, and advanced lab instruments. Each article is paired with a browser-based simulator so theory and applied technique stay tightly coupled. Start with the refractometer workflow table below, then work through the supporting articles for calibration tips, error sources, and per-instrument worked examples.

Refractometer workflow: seven steps

Step Action
1 Clean the prism with the supplied solvent and a soft tissue. Any residue affects the half-shadow boundary.
2 Apply one small drop of contact fluid (refractive index 1.79–1.81; di-iodomethane saturated with sulphur) directly to the prism.
3 Place the stone polished-table-side-down so it rests fully in the fluid. The table must lie flat; wedge thin stones with a small fluid pool.
4 View through the eyepiece under sodium (589 nm) or filtered white light. Adjust the eyepiece focus until the scale and shadow boundary are crisp.
5 Rotate the stone through 180° in 45° increments. Singly refractive stones show one stationary shadow; doubly refractive stones show two boundaries that move (the birefringence).
6 Record the highest and lowest readings to ±0.002. The difference is the birefringence; the midpoint is the spot reading.
7 Cabochons and rough surfaces use the spot method: a tiny droplet on the prism contacts only the dome. Read the point at which the bright field switches to dark.

The standard prism saturates at ~1.78; readings beyond that are over-the-limit (OTL). For OTL stones (diamond, zircon, demantoid, sphene, some garnets, cubic zirconia) use the Hanneman / Hodgkinson short-cut or the refractometer simulator.

All equipment articles

Gem Testing Equipment Overview

Introduction to gemmological instruments, testing workflow, and which tool to use for different identification tasks.

beginner

The Refractometer

Refractive index measurement using the gemmological refractometer, including faceted and cabochon techniques, birefringence reading, and troubleshooting.

beginner

Advanced Laboratory Instruments

Diploma-level awareness of FTIR, UV-Vis-NIR, Raman, EDXRF, LA-ICP-MS, and photoluminescence spectroscopy: what each detects and its key gemmological applications.

advanced

Chelsea Colour Filter

Using the Chelsea Colour Filter as a chromium discriminator for gem identification, covering the species reaction table, lighting requirements, and diagnostic limitations.

beginner

Conoscopic Observation and Interference Figures

Using convergent polarised light to observe interference figures that reveal a gemstone's optic character (uniaxial or biaxial) and optic sign – an advanced polariscope technique.

advanced

Diamond Screening Instruments

Thermal conductivity probes, electrical conductivity testers, DiamondView, DiamondSure, and reflectance meters – the tools used to screen loose diamonds and detect synthetic and simulant materials.

intermediate

Other Testing Tools

Chelsea filter, thermal conductivity probes, specific gravity measurement, and other supporting gemmological instruments.

beginner

Specific Gravity Measurement

Hydrostatic weighing and heavy-liquid methods for measuring specific gravity – the density-based property used to identify and confirm gem species.

beginner

The Dichroscope

Using the dichroscope to observe pleochroism in coloured gemstones, distinguishing dichroic from trichroic materials.

beginner

The Microscope

Using the gemmological microscope for inclusion study, illumination techniques, and immersion methods to identify gems and detect treatments.

intermediate

The Polariscope

Using the polariscope to determine optic character (isotropic vs anisotropic), detect anomalous double refraction, and observe interference figures.

beginner

The Spectroscope

Using the spectroscope to observe absorption spectra, identifying characteristic patterns for gem species and detecting treatments.

intermediate

The UV Lamp

Using ultraviolet light for fluorescence testing, distinguishing LWUV from SWUV reactions, and safety considerations.

beginner

Try the browser-based simulators

Every instrument in this module has an interactive companion widget. Run real measurements without owning the hardware: