Next to the refractometer and the loupe, the microscope is probably the most-used instrument among gemologists. Its obvious use is for magnification of internal and external features of gemstones, giving the gemologist extra information about the identity and quality of the stone.
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Gemologists usually use binocular microscopes with zoom capabilities ranging from 10 to 90 times magnification. In most circumstances, a magnification of 45x is sufficient for day-to-day operation. These binocular zoom microscopes are the most expensive pieces of identification equipment used in the standard gemological laboratory.
When handled with care, they will last a lifetime and are well worth the investment.
Some binocular microscopes do not have a zoom feature and these are far less expensive. Typically, the magnification ranges go from 10 to 30 to 60 times (in steps). For these steps of magnification, the oculars (eyepieces) need to be exchanged.
A good gemological microscope should at least be equipped with a light field and a darkfield, an overhead illumination and preferably an adjustable iris. An integrated stone holder is a good choice as well.
Types of illumination
Most gemological microscopes provide a whole range of illumination and associated techniques which all have their characteristic uses.
Good gemological microscopes will have an adjustable iris diaphragm that will enable you to reduce the light source by closing the diaphragm to just a small pinpoint opening.
This makes it easier to spot curved striae and other structures inside the gemstone.
Opening the diaphragm to just less than the diameter of the gemstone works as a "shadowing technique".
Light field illumination
With the light entering directly from below, low relief inclusions and curved striae are seen as dark objects against a bright background. This if often used with pinpoint illumination.
Darkfield illumination is probably the most-used type of illumination in gemology.
A dark plate covers the bright light illumination, causing the light to be reflected before reaching the stone. Inclusions will stand out bright against a black background when using this technique.
Overhead lighting usually involves fluorescent lighting and is used to inspect the external features of gemstones, such as polishing marks, pitting and other marks that are found on the surfaces of gemstones.
When a sheet of tissue paper or any other diffusing object is placed over the bright light illumination, the light will become scattered and softened. This makes it easier to observe broad color zoning and color banding.
A white frosted plastic or glass plate is best for this technique. A blue diffusor may come in handy when looking at yellow to orange stones, as the blue will counteract the yellowish hue in most light sources.
When you use tissue paper, make sure the tissue doesn't get heated by the light as that is a potential fire hazard.
Horizontal illumination (or penlight illumination) is used to direct horizontal light into the stone. This causes gas bubbles and small inclusions to stand out brightly.
A fiber optic light could also be used for this.
With polarization filters in place, the microscope becomes a polarizing microscope (a polariscope with the benefit of magnification). This is very useful when looking at twin planes, strain, pleochroism, and for determining optic character (and sign) and for distinguishing synthetic from natural Quartz.
A very cost-effective setup (about USD 10) involves a polarizer placed over the iris and an analyzer sticky-taped under the pod.
Fiber optic illumination has the big advantage of letting you direct and concentrate light where it is most needed to view inside the gemstone.
Generally, a 150 watt fan-cooled illuminator is used as the lighting source. These light sources are highly portable and can be used for other instruments (like the refractometer and the spectroscope) as well. A highly suggested add-on for your laboratory.
An opaque, thin object (like a credit card) is inserted in the light path. This introduced object causes diffraction on the light beam and the 3 dimensional image of inclusions is greatly increased..
Growth structures such as twinning and curved striae are much more easily observed with this technique.
- Mark VII Gemolite Microscope Owner's manual - GIA Gemological Instruments
- Ruby & Sapphire (1997) - Richard W. Hughes
- Gemmology 3rd edition (2005) - Peter Read