Function

An immersion cell is a small, optically clear glass dish used in gemmology to suspend a stone in a liquid whose refractive index approaches that of the stone itself. Once the surface reflections from the facets are suppressed by index matching, structural features inside the stone become visible to the microscope or loupe. Curved colour zoning in synthetic corundum, hexagonal zoning in natural ruby and sapphire, growth-tube patterns in flux-grown synthetics, doubling in tourmaline, and silk in heated versus unheated corundum all read more cleanly under immersion than in air.

Construction

Cells are typically rectangular or cylindrical, ground from optical glass, with parallel faces to avoid distortion. Standard sizes range from roughly 10 by 10 by 25 millimetres for small loose stones up to several centimetres for mounted pieces. The base is flat and clear. A loose-fitting glass cover is sometimes used to limit evaporation of volatile liquids.

Common immersion liquids

Liquid choice depends on the refractive index of the stone and on safety considerations. Methylene iodide, RI 1.74, was historically the workhorse for corundum and most coloured stones. It is now restricted in several jurisdictions because of toxicity. Bromoform, RI 1.59, and clove oil, RI 1.54, remain in regular use. Distilled water, RI 1.33, suffices for surface-feature work on lower-RI material. Glycerine, RI 1.47, is a safer alternative for student use.

Practical use

The stone is set table-down or pavilion-down in the cell and the chosen liquid is poured to cover. Examination is conducted with darkfield or diffused transmitted illumination through the base, viewed from above with a stereomicroscope or loupe. Lotus Gemology, GIA, AGL and most major coloured-stone laboratories rely on immersion as a routine first-pass screening tool, particularly for separating natural from synthetic corundum and for documenting heat-treatment indicators.

Care

Cells are cleaned with appropriate solvents, usually alcohol or acetone, then dried. Methylene iodide darkens with age and exposure to light and should be stored in dark bottles. Spent liquids are disposed of through laboratory waste channels, not down the drain.