Microscope

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Revision as of 12:08, 2 October 2006 by Doos (talk | contribs) (Oblique illumination)
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The microscope is next to the refractometer and the loupe probably the most used instrument amongst gemologists. It's obvious use is for magnification of internal end extrenal features in gemstones which gives the gemologist extra information about the identity and quality of the stone.

Basic

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Gemologists usually use binocular microscope with zoom capabilities ranging from about 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 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 atleast have lightfield, darfkfield and overhead illumination with an adjustable iris. A stoneholder is also preferable.



Types of illumination

Most gemological microscopes provide a whole range of illumination and associated techniques which all have their characteristic uses.

Pinpoint-source illumination

Pinpoint-source illumination

Good gemological microscopes will have an adjustable iris diaphragm which will enable you to reduce the light source by closing the diaphragm to a small pinpoint opening.

This makes it easier to spot curved striae and other structures inside the gemstone.

When you would open the diaphragm to just less than the diameter of the gemstone, this will also work as a "shadowing technique".



Lightfield illumination

Lightfield 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

Darkfield illumination

Darkfield illumination is probably the most used type of illumination in gemology.
A dark plate covers the brightlight 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 illumination

Overhead illumination

Overhead lightning usually involves fluorescent lightning and is used to inpect the external features of gemstones, like polishing marks, pitting and other marks that are found on the surfaces of gemstones.


Diffused illumination

Diffused illumination

When a sheet of tissue paper or any other diffusing object is placed over the brightlight illumination the light will become scattered and softened. This makes it easier to observe color zoning and color banding.

When you use tissue paper, make sure the tissue doesn't get heated by the light as that is a potentional fire hazard.


Horizontal illumination

Horizontal illumination

Horizontal illumination (or penlight illumination) is used to direct horizontal light into the stone. This causes gass bubbles and small inclusions to stand out brightly.

A fiber optic light could also be used for this.


Polarized illumination

Polarized illumination

With polarization filters inplace the microscope becomes a polarization microscope (a polariscope with the benifit of magnification). This is very useful when looking at twinplanes, determine optic character (and sign), strain, pleochroism and distinguishing synthetic from natural Quartz.

A very cost effective setup (about USD 10,00) involves a polarizer placed over the iris and an analyzer sticky taped under the pod.


Oblique illumination

Oblique illumination

Fiber optic illumination has the big advantage that you can direct and concentrate light where it is most needed to view inside the gemstone.

Generally a 150 Watt fan cooled illuminator is used as lightning source. These light sources are highly portable and can be used for other instruments (like the refractometer and the spectroscope) aswell. A highly suggested add-on for your laboratory.


Shadowing technique

Shadowing technique