Gemology tools : The Microscope, A Guide for Gemologists

Gemology tools

The Microscope, A Guide for Gemologists

How Much Magnification do You Need?

You can identify most gems with just 40 power. However, you can't identify all gems without more magnification. While 80 power has long been considered adequate, new and higher quality synthetics sometimes require even more magnification.

While you only need a 10X loupe for gem grading, a microscope will give you a larger field of view and a brighter image. It will also show alterations like fillings, dye concentrations, assembled stones, and diffusion treatments.

Many gemologists use their microscopes for taking photographs for their appraisals. It's also a useful sales tool. For example, you can show customers identifying features or why the prongs on a jewelry piece need replacing. In addition, seeing a gem under magnification is impressive. Gemstone photomicrography can produce fantastic images.

Microscope Features

Lenses

Objectives are the lenses on the bottom. Oculars are the ones you look through. A microscope's power is determined by multiplying the value of these lenses. For example, if you have 3X objectives and 20X oculars, the power is 60. A magnification of 60X is expressed as 60 power. That means the image you see is sixty times larger than you would see unaided.

The quality of the lenses affects the quality of the image you see. However, buyers may find it difficult to judge lens quality. In better quality microscopes, the difference in image quality is subtle. Often, you'll find a substantial price difference for just a small improvement. Inexpensive scopes, on the other hand, often have plastic lenses. The difference in clarity between good glass lenses and plastic is substantial.

One easy-to-judge feature is lens size. The larger they are, the brighter the image. Oculars range from around 10 to 30 mm. Try to get a microscope with the largest lenses you can afford. If your eyesight is less than perfect, this is especially important.

Pod

The pod is the element that holds the optics. There are three features to consider in a pod:

• First, how far will it travel? The higher you can raise the pod, the larger the piece you can examine. A binocular microscope designed for viewing slabs, like those used by metallurgists, will only travel a short distance. Some of these won't rise high enough to examine the center stone in a large ring.
• Second, do you need a rotating pod? If you want to show customers things you've found with the microscope, you do. Moving the whole microscope so the customer can see will cause enough vibrations to lose focus. Your only other realistic option is to ask the customer to take your seat. However, that may require them to come behind your counter! So, now you know why a microscope with a rotating pod is a good idea.
• Finally, consider removable oculars. You can remove the oculars on most microscopes. This allows you to change magnification or add accessories, such as a camera or spectroscope.

If possible, choose a microscope that has all three features for the greatest versatility.

Focusing

Every microscope has a focusing method. Some have two controls, one for coarse and one for fine adjustments. What's important for the focusing mechanism is smoothness and security. The smoother the control works, the easier it is to bring small details into focus. It must also stay in place without any play in the movement.

Most microscopes have one adjustable ocular. This allows you to fine tune the focus for each eye. Follow this procedure for adjusting the oculars:

• Set the microscope to its highest power.
• Turn on an overhead light.
• Close one eye and look through the non-adjustable ocular.
• Focus on something near the center of the stage.
• Change eyes and look through the adjustable ocular.
• Adjust the focus of the ocular, not the objectives.
• Open both eyes. Verify that everything is sharp and three-dimensional. If not, repeat the above steps.

Stage

This is where the viewing takes place. The stage should be large enough for any size gem or jewelry piece. While the stage size is important, the amount of travel in the pod has more effect on how far up you can focus.

Base

The base should hold the microscope firmly. If you intend to rotate the pod so customers can view the gems, this is especially important.

Changing Magnification

Examining a gem begins with low-power magnification, usually 20X. This allows you to see more of the gem and gives you a better understanding of its overall characteristics. When you find inclusions, you need higher magnification to see them in detail.

There are two ways to change the magnification:

• The most common method involves changing the lenses. A typical microscope will have 2X and 4X objectives. With 10X oculars, you can change from 20 to 40 power simply by rotating the objectives. For higher magnification, you would change the oculars. By switching to 20X oculars, you can have 40 and 80 power magnification.
• Zoom microscopes are also available. With these, you can change magnification by adjusting a knob. While this is a very nice feature, it will add considerably to the cost of the microscope. If you're considering a zoom microscope, make sure it stays in focus while changing the power. Adjusting the focus while changing magnification is so awkward that it takes away the advantage of having a zoom.

Stone Holder

This is a pair of tweezers that attaches to the base of the microscope. It'll hold a gem far steadier than your fingers, which is important at high magnification. Also, it'll keep the jewel in position, so you can show it to someone else.

Microscopes designed specifically for gemology come with stone holders. If your scope doesn't come with one, you can order a stone holder from a third party. It replaces one of the stage clips. You can change it simply with a screwdriver.

Lights

Most gemological microscopes come with overhead and substage lighting. Although you'll use these frequently, they won't meet all your needs. A third, movable light will do much more to bring inclusions into view than the built-in lights. This third light can be a simple penlight, a desk lamp with a flexible arm, or a fiber optic unit. Beginning gemologists should understand that they shouldn't rely on the built-in lights alone. That will make the work much more difficult.

Many microscopes also have apparatuses for darkfield viewing and adjustable irises. While great conveniences, they're not necessities. You can make accessories that work as well or better than the built-in features. Although they may not be quite as easy to use, they'll perform admirably.

Lighting Techniques

Lighting is one of the most important elements of a microscope. The inclusions you're searching for have a wide range of optical properties. Therefore, you'll need bright light and the right magnification to spot them. Some, like the carbon inclusions in diamond, are opaque and easy to see. Although you'll frequently find crystal inclusions in many gems, they're not all as easy to see as those in diamonds. If the inclusion's refractive index (RI) differs considerably from the host material's, it will stand out in relief. You can distinguish such inclusions fairly easily. However, if the inclusion and host material have nearly the same RI, like a ruby crystal in a ruby, you'll have a harder time. Small fingerprint inclusions and fractures are also difficult to see. Since they're essentially invisible, you'll only see them when light reflects off them.

To spot inclusions in a gem, you must try an assortment of lighting techniques. What works well for one kind of inclusion will hide others. Since you don't know what inclusions a gem holds when you start, you need to use a variety of techniques in your exam.

Overhead Light

Overhead light is best for observing surface characteristics. However, it's also an excellent way to begin your examination, since it gives a good overall view of your gem.

Substage Light

Lighting a gem from beneath will show more inclusions than overhead lighting.

However, looking into the light creates its own problems. To overcome this, gemologists use a variety of techniques in combination with substage lighting.

Diffused Light

Light is diffused by passing through a piece of frosted glass, a piece of paper, or facial tissue. The even, luminous background helps make color zoning and color banding easier to see.

Darkfield

A highly effective method is to place a black object directly under the gem so that light enters the gem from the lower sides. This makes some inclusions stand out vividly.

Some scopes have a disk built in for this purpose. If your microscope doesn't, making some black disks is a simple thing. Make different sizes, since what works well for large gems won't work as well for small ones.

Brightfield

Here, the outer part of the stage is dark, but light comes up the center. If your microscope has an adjustable iris, close it until the opening is smaller than the gem. Otherwise, you can make simple baffles by cutting a small hole in a black disk and placing it under the gem. A disk with a small hole, about 2.5 mm or so, can make this technique particularly effective. Move the disk around so the thin band of light passes through different areas of the stone.

This technique is good for finding color banding and liquid-filled inclusions. Also, it's one of the most effective techniques for discovering small, hard-to-find inclusions. However, it will also mask some inclusions. Therefore, be sure to inspect the gem thoroughly with other lighting as well.

Oblique Lighting

This term refers to illuminating the gem from any direction other than directly from the top or bottom. It's one of the most useful techniques for finding inclusions:

• First, place a black disk under the stone.
• Next, get an external light source, like a penlight or small flashlight, flexible arm desk lamp, or fiber optic light.
• Then, move the light source around so light enters the gem from several directions.

You'll be amazed at how easy it is to find inclusions this way.

Where is the Inclusion?

Occasionally, you may wonder if an object is on the surface of a gem or inside it. Use one of the following methods to tell the difference:

• Adjust the light so it reflects off the surface. If the object is internal, you'll see an unbroken, mirror-like reflection. If it's on the surface, it'll stand out above the reflection.
• Pick a reference point on the surface of the gem, like a scratch or facet junction. Rotate the stone and watch both elements. If they move together, the object is on the surface. If it's internal, it'll move more or less than your reference point.
• Adjust the focus. If the surface and object don't come in focus at the same moment, the object is internal.

Microscope Techniques

Before you begin, thoroughly clean the gem. Not only will dust and (human) fingerprints interfere with your view, they're also difficult to distinguish from inclusions.

Basic Procedures

Starting with low magnification and overhead lighting, carefully survey the gem. Take note of anything you see, such as fracture types, sharp or rounded facet edges, abrasions, and lapidary quality. These surface characteristics can be important clues later.

If you find any inclusions, go back and bring them into view. Use higher magnification and any lighting technique necessary to see them clearly.

Now, switch to diffused substage light. At this point, you're looking primarily for color zoning and growth lines. If you find any inclusions, take the necessary steps to see them clearly.

Next, place a black disk under the gem and use the oblique lighting technique. Illuminate the gem from every possible direction. Note any inclusions you find. Again, take the necessary steps to bring them into clear view.

Immersion Procedures

If you've found the necessary information through the basic procedure and are certain the stone is one piece, skip this step.

Immersion simply means placing a stone into a glass vessel filled with a refraction liquid. This reduces reflections, making the gem's features easier to see. Also, it aids in distinguishing inclusions, color zoning and banding, and highlights separation planes.

This procedure is often the best way to distinguish assembled stones. Your first clue you're viewing an assembled stone is seeing bubbles in the glue between layers. However, you may find distinguishing them from other inclusions difficult. Under immersion, crowns and pavilions with different colors or RIs stand out vividly. Colored cement layers are also easier to distinguish.

To use the immersion method, set the microscope for diffused illumination. Place a small glass vessel on the stage filled about half way with liquid. Be careful not to start with so much that it runs over the top when you add the stone. Place the stone in the vessel and add more liquid if needed.

Plain water will do. However, the closer the liquid's RI comes to that of the gem, the more effective the procedure. Although high RI liquids, like methylene iodide, work well, they're toxic.Please note, the heat from the microscope lighting will cause them to release more noxious fumes than at room temperature. You can use safer substitutes, such as water and vegetable oils. See the chart below to find a suitable immersion fluid for your test gem.

Begin with low magnification and increase the power as needed. Start with the stone sitting on its table and turn it as needed.

The immersion technique involves the additional steps of setting up and cleaning the gem when finished.

Refractive Indices of Common Refraction Fluids

Immersion Fluids	Refractive Index
Water	1.33
Alcohol	1.36
Corn oil	1.47-1.48
Olive oil	1.44-1.47
Glycerin oil (glycerol)	1.47
Almond oil	1.45-1.47
Clove oil	1.53-1.54
Wintergreen oil	1.54
Anise oil	1.54-1.56
Cinnamon oil	1.59-1.62

 

Techniques for Difficult Stones

You'll typically find the preceding microscope techniques suffice for finding identifying inclusions. However, you'll also find it's difficult to find inclusions in some stones, particularly synthetics.

Try these three techniques for finding exceptionally small inclusions:

• Try the oblique lighting method again, but with higher magnification. As you go to higher powers, the inclusions appear larger. However, you see less of the stone. So, raise the magnification in steps, rather than going directly to your highest power.
• With substage light, place a disk with a small (2.5 mm) hole under the gem. Move the disk around so the thin band of light passes through different areas of the stone. Rotate the gem and progressively increase the magnification.
• Immerse the gem and examine it from several angles and with increasing magnification