How to reduce the surface reflections from a gemstone

by Peter Read

 

In a previous issue I mentioned in this column how the inclusions in a gemstone can be used as an aid to identifying the stone. One of the difficulties in viewing the interior of a gemstone with amicroscope is caused by light rays reflected back from the stone’s facets.

 

A way to reduce these reflections is to inject the illuminating light into the side of the gem, so that any inclusions are highlighted against the dark background of the stone (on a suitably equipped microscope, this takes the form of darkfield illumination).

An even beeter method, which both reduces surface reflections and helps to illuminate the interior of the gem, consists of immersing it in a liquid having a refractive index close to that of the stone (e.g. for emeralds, bromoform with an R.I. of 1.59, or for rubies and sapphires, methylene iodide with an R. I. of 1.74). The surface reflections are reduced in this case because the reflectivity of a polished surface is related directly to the refractive indices of both the surface material and its surrounding medium (normally air).

This relationship is shown in the simplified Fresnel formula, Reflectivity = (R.I. of gem – A)² ÷ (R.I: of gem + A)², where A is the reflectivity of the surrounding medium (which, for air, is 1.0).

For a ruby, the reflectivity of its surface facets in air is as high as 8% (it’s 17% for diamond), but immersing a ruby in methylene iodide reduces this to a mere 0.002%. Even water, with an R.I. of 1.33 will reducer the surface reflectivity of a ruby to 2%.

When using the immersion technique to view gemstone inclusions with a microscope it’s convenient to mount the gem in stone tongs so that it can be orientated for all-round inspection. Unfortunately, with the standard microscope it’s necessary to use a shallow immersion cell so the stone tongs can be kept at a reasonably shallow angle to avoid masking the view of the gem.

 

Horizontal immersion microscope SYNTEST F for reducing reflections on gemstones in microscopy

One way out of this difficulty is to mount the microscope head so that the objective lens views the gem in a horizontal rather than vertical direction. This then allows the stone tongs to be positioned vertically, and the gem to be immersed and rotated in a deep cell. Horizontal-objective microscopes of this type have been manufactured and promoted mainly by the German gemmological instrument company System Eickhorst.

To minimize the expense of purchasing a complete horizontal-type microscope, System Eickhorst has introduced a new “Horizontal Immersion Base “SYNTEST”.

The stand is fitted with a circular clamp which enables it to be used with the head from an existing microscope (provided that this is removable and has an appropriate body diameter).  The stand is fitted with a light source that can be pivoted to give incident, darkfield or lightfield illumination.

Alternatively, the base can be provided with a choice of stereo heads, with fixed magnifications of 10x and 30x, zoom models with a range of 7x to 60x and a ZEISS trinocular zoom model (for camera attachment) with a ZOOM range of 6.5x to 80x.

The overall magnifications of all these models can be doubled by fitting 20x eyepieces. For the zoom heads, the magnification can be doubled again by means of an objective-lens adaptor of 2x.

For anyone who hasn’t already got a microscope, System Eickhorst also markets a vertical “Diamond Darkfield Stand”. This can be used with the same heads as the horizontal stand and is fitted with an innovative darkfield.

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