RI liquid, in standard gemmological practice, is the high-refractive-index liquid placed between a gemstone and the prism of a refractometer to create the optical contact necessary for an accurate refractive-index reading. The liquid serves a single function — to bridge the air gap that would otherwise produce total internal reflection at the prism surface — and its own refractive index must be higher than that of the stone being tested but compatible with the refractive index of the prism. The standard formulation is a saturated solution of sulphur in di-iodomethane (also called methylene iodide, CH₂I₂), with an RI of approximately 1.79 to 1.81 at 20 degrees Celsius. The liquid is supplied by a small number of laboratory chemical houses and is one of the few essential consumables of the gemmological bench.

Use

A drop of the liquid is placed on the polished prism of the refractometer. The polished face of the test stone is pressed firmly against the prism so the liquid forms a thin, continuous film between the two surfaces. The instrument is illuminated through the back of the prism, and the shadow edge or edges read against the calibrated scale visible through the eyepiece. The technique is straightforward but requires practice — the stone must be oriented with a polished facet flat against the prism, the lighting and focus must be set correctly, and the gemmologist must learn to interpret the shadow edges that correspond to the ordinary and extraordinary refractive indices of birefringent species.

Removal of the stone and cleaning of the prism with a soft tissue or chamois follows each test. The prism must be cleaned thoroughly between stones to avoid contamination of the next reading; residue from a previous stone or from skin oils on the test specimen can produce shifted readings on subsequent tests.

The liquid must wet the prism and the stone uniformly. A bubble or a contaminated drop produces a shifted or blurred shadow edge that gives an inaccurate reading. Reputable laboratories renew the liquid bottle as soon as the optical performance degrades; old or contaminated liquid is the most common cause of refractometer error in routine work, and trained gemmologists check the liquid against a known-RI reference stone (typically a synthetic spinel of known index) at the start of any testing session.

Hazards and handling

Sulphur-saturated di-iodomethane is toxic, volatile, and light-sensitive. Inhalation of the vapours over time is harmful, and prolonged skin contact should be avoided; the gemmological supplier's safety data sheet should be consulted before first use. The liquid darkens on exposure to ultraviolet and visible light, with iodine being released in the process and the refractive index drifting as the composition changes; storage is in a dark glass bottle, kept tightly closed and out of direct light. Spillage is cleaned with absorbent material and disposed of as chemical waste under local regulations.

The high refractive index limits the refractometer to stones with RI below approximately 1.81 (the prism's RI sets the practical upper limit). Stones with higher RI — diamond at 2.42, garnet of pyrope-spessartine composition, cubic zirconia at 2.16, synthetic moissanite at 2.65 to 2.69 — produce a negative reading and must be identified by other methods, typically the immersion method using calibrated RI test sets, or by direct measurement using a reflectivity meter or a Brewster-angle device.

Alternatives and contemporary practice

The standard sulphur-iodide formulation has been challenged occasionally by efforts to find a less toxic substitute, but no alternative has both the right refractive index and the right wetting and viscosity behaviour to match the original. Some laboratories use lower-RI liquids for specific testing protocols, but the standard remains the workhorse of routine gemmological identification. Modern advanced laboratories supplement the refractometer with Raman and infrared spectroscopy for definitive identification, but the refractometer and its companion liquid remain the first-line tool of the bench gemmologist.

Further reading

• GIA Gems & Gemology — articles on refractometer use and gem identification
• International Gem Society — references on refractometers and RI testing
• ICA — gem identification references