An RI test set is a graduated collection of calibrated refractive-index liquids, typically spanning the range from approximately 1.40 to 1.81 in increments of 0.005 to 0.01, used to determine the refractive index of a gemstone by the immersion method. The set complements the refractometer, which has a practical upper limit at the RI of its prism (around 1.81), by providing identification capability for stones whose RI exceeds that limit and by offering a non-instrumental method for routine identification of small or oddly shaped specimens. The leading commercial supplier is Cargille Laboratories of New Jersey, whose Cargille liquids are the trade reference standard, supplied in small bottles with calibrated RI values to four decimal places at a stated reference temperature.

How the method works

The Becke-line method is the underlying principle. A small piece of the stone — or, for testing without sample preparation, the polished stone itself — is immersed in a vial of liquid of known refractive index and viewed under diffuse light through a microscope or a strong loupe. When the stone's RI matches the liquid's, the stone's outline disappears against the background; the stone is said to be indistinct in that liquid. When the indices do not match, a faint bright halo — the Becke line — forms at the boundary, and as the microscope focus is raised, the line moves toward the medium of higher refractive index. The gemmologist therefore reads the direction of Becke-line motion to determine whether the stone's RI is higher or lower than the liquid's.

By immersing the stone successively in liquids of progressively closer RI, the gemmologist brackets the stone's index between two adjacent liquids and reads the value to within the resolution of the set. For routine work, this method is fast and avoids the need to polish a flat for the refractometer. With a complete set covering the gem range, the typical accuracy is plus or minus one or two thousandths in refractive index, sufficient for routine species identification.

Use cases

The test set is essential for identifying high-RI species — diamond at 2.42, pyrope-spessartine garnets up to 1.81, cubic zirconia at 2.16, synthetic moissanite at 2.65 to 2.69, sphalerite at 2.37 — which read off the refractometer scale. It is also useful for thin slices, small fragments, polished pebbles, and rough stones whose surfaces cannot easily be brought into optical contact with the refractometer prism, and for fast preliminary screening of mixed parcels.

The liquids are typically bromine-based, halogenated, or fluorinated organic compounds, selected for stable refractive index and acceptable handling characteristics. They are calibrated by the supplier and are accompanied by certificates of value at standard temperature; precise work requires temperature correction using the supplier's data, with typical thermal coefficients of about minus 0.0004 per degree Celsius.

The set is also used in mineral microscopy and in petrography for thin-section work, and the same supply chain serves both communities. Some laboratories maintain dual sets — one for gemmology, one for thin-section work — to avoid cross-contamination from sample residues that might affect optical identification of fine mineralogical features.

Handling

RI liquids vary in toxicity and volatility. They should be stored in dark glass, kept tightly closed, and used in a ventilated workspace. Direct skin contact and inhalation should be avoided, and the supplier's safety data sheet should be consulted before first use. Decanting from the storage bottle into a small working vial reduces contamination of the bulk supply, and gemmologists often work with a series of small dishes rather than dipping into the supplier's bottles directly.

The set must be checked periodically for drift. Liquids age by evaporation of volatile components, by photochemical decomposition (hence the dark-glass storage), and by absorption of moisture from humid air. A drift of a few thousandths in RI is enough to compromise routine identification work, and a quality-conscious laboratory tests its set against known-RI reference stones at the start of any extended testing session.

Further reading

• GIA Gems & Gemology — articles on refractive-index testing and gem identification
• International Gem Society — references on Becke-line and immersion methods
• ICA — gem identification references