Concept

An immersion refractometer determines refractive index by comparing the refraction of a stone immersed in a series of liquids of known index. When the stone is suspended in a liquid of matching index, the boundary between stone and liquid disappears, and the index can be read from the calibrated liquid. The technique works on rough, fragments, and material whose surfaces are too rounded or pitted for a contact refractometer.

Forms in use

The classical Becke-line method uses immersion under microscope observation. The stone is placed in a drop of liquid on a slide. As the focus is racked up and down, a bright line, the Becke line, moves toward the medium of higher refractive index. Successive liquids are tried until the line vanishes. The liquid index at vanishing point equals the stone index.

Direct-reading immersion refractometers, less common in modern practice, mount the immersion cell on a graduated rotating stage and read the index off a scale. The Duplex II and similar contact refractometers, by contrast, do not use immersion but use an optically dense glass hemicylinder.

Use cases

The instrument is most valuable for inclusion-rich, heavily included or rough stones whose flat polished facets are absent. Petrographic and mineralogical laboratories use immersion routinely for thin-section work and for identifying small crystal fragments. In gemmology, it is a backup technique when contact refractometry fails.

Limitations

Indices above roughly 1.74, the upper limit of methylene iodide, require specialised liquids such as saturated solutions of mercuric salts in methylene iodide, or substituted refractive-index oils, all of which carry handling concerns. For diamond, RI 2.42, immersion refractometry is not practical and other identification methods are used.