(How to) Use it seriously
(via Gemmology Queensland, No.3. No 6, July 2002) Trevor Linton writes:
Carry it all the time and use it as much as possible every day. A 10x lens increases knowledge. A 10x is the first and possibly the most serious instrument used by both the amateur and professional when investigating a gemstone’s features and information as to its origin.
The hand lens, (10x or times—10) is a combination of lenses that provides magnified detail to the observer. It is the most important instrument you can use when an unknown gemstone is given to you either for purchase or for identification. Do not dismiss this step of the investigation into a gemstone during your quest for information, as the hand lens is the only instrument that provides an overall view of a gemstone. The 10x hand lens, in experienced hands, and under quality lighting, can provide essential identification features on 90 percent of gemstones: by noting hardness, refractive index, birefringence, dichroism and dispersion as well as indications of the gemstone’s origin from inclusions and many methods of man’s treatment.
The quality of a 10x hand lens is usually dependant on price, which can vary from A $10 to $320 depending on its type, source, manufacturer and country of origin. There are three main lens combinations that govern the quality of the lens: the plano-convex doublet, the achromatic doublet, and the compound triplet. Each has its advantages and disadvantages.
The first type, the economic doublet has two simple crown glass lenses facing each other at approximately two thirds of their focal length. This combination of lenses provides a 40 percent field of vision that is in reasonable focus and has a relatively flat field. When observing a sheet of squared paper with the doublet, an image appears. This is suitable for rough investigation of stones, but when serious images of a flat facet are required, it leaves a lot to be desired……as straight lines are curved, only the center is in focus as the observed field is curved through a doublet. The colors red and blue are not in focus with green. The advantage for the doublet is economy. A lot of detail can be gathered for a small outlay of dollars with a doublet. Chromatic aberration is usually not a great influence with gemstones as only highlights are refracted in to the differing points of color focus that produce color fringing. Most gemstone observations are in gemstones of one color or colorless gemstones.
Increased color resolution is achieved with a flint glass lens cemented to the inside a double sided convex crown glass lens forming an achromatic compound lens that corrects for the chromatic aberrations that defocus colors in a lens system. This second type of lens system produces straight lines over most of the field of view. An achromatic doublet is a very good, yet still economic 10x system of lenses.
The third lens type, the triplet lens, is a solid system of glass lenses using three layers of glass cemented together forming an aplanatic (in a plane) triplet. The image of a flat plane forms as a plane at the point of focus. This is the best system for hand lenses, when correctly designed and used under normal observation. A triplet has minimal spherical and chromatic aberration and a flat field.
When purchasing a hand lens, buy one that is better than you initially need. If you buy the best triplet lens, use it to your advantage. That s what the whole project should be about. You have to achieve quality observation. There is little advantage in using a quality lens without good lighting and knowledge of how to use both.
When using the hand lens there are several basic rules, and many techniques to develop:
- Spectacles should not be worn unless severe optical defects are present within eyes.
- Choose a good quality light source, with a very narrow beam of high intensity light directed on to your work area and not spilling light towards your eye or the background.
- Work over a soft white cloth that allows easy pick up of gemstones with tweezers. This soft cloth also prevents dropped gemstones from bouncing off the work area on to the floor.
- Clean the sample free from dust and grease (fingerprints) with a lint free cloth. A quality glasses cleaning cloth is suitable.
- Always pick up gemstones with tweezers after cleaning.
- When using the right eye hold the hand lens in the right hand about 25mm in front of the eye, with the index finger primary knuckle resting on the neck.
- Keep both eyes open. This can occur with experience. When the closed eye lid flutters letting light in there is a complimentary iris movement in the open eye.
- Holding the lens close to the eye with one hand, bring the gemstone close with the other hand and rest both hands together for stability. Position the stone 25mm in front of the lens with the gem’s surface in focus, under the light.
- Ensure the lens is straight in front of your eye and is not twisted. Looking off the central axis will produce ‘coma’ color fringing on all images that severely limit quality observing.
- Work systematically around the surface of all faceted gemstones before being drawn into the gems interior. There are many features available for observation, especially around the girdle. Gemstones such as diamond reveal their true nature on the girdle, with natural crystal surfaces left on opposite side of the girdle as the cutters try to achieve maximum weight from the rough stone. The treatment of the girdle or how it was formed is a good guide to the cutting quality and whether care was taken during the gem’s manufacture.
- Do not neglect other features such as flat facets and sharp facet edges that indicate hard gemstones. Concave facets or molding marks at the girdle indicate cast glass. Reflect a fluorescent tube image from the facet for an image of a straight flat facet. Surface luster of facet edges chips indicates the refractive index of the gemstone. Try looking at glass or quartz and comparing surface chips with those on sapphire or diamond. Yes diamond does chip before it cleaves in to two diamonds.
The history of a gemstone’s growth, and subsequent heat and coloring activities induced by man, occurs in the gem’s inclusions. Your 10x hand lens will reveal many of these features under these proposed lighting techniques of use.
Inclusion types have a greater listing than there are gemstone types. Quartz alone has 550 officially listed inclusion types. Quartz is a low temperature forming gemstone; it is of the last to crystallize and includes many other gems that form before it does. Pegmatites in which quartz forms have plenty of liquid and gas so there are many of these inclusions in the ‘veils’ that form within fractures of quartz crystal.
Igneous (volcanic) gemstones can be easily distinguished from metamorphic gems by the individual suites of inclusions within each type. Man-made gems may be chemically identical with those of nature, but man’s techniques of manufacturing these gems create characteristic inclusions that are identifiable with your 10x hand lens.
An interesting feature is found on many older gemstones when a flat plane of air bubbles are glued in a layer between the crown and pavilion of a doublet. These are very easily found with a hand lens. Dispersion of color at individual facets is a good indicator of high refractive index and hardness.
Other hand lenses in use through the industry, such as the Coddington lens and the darkfield loupe have specific features for overcoming specific problems. For example, The Coddington lens is a solid glass cylinder with a restricting light aperture half way at its focal point. This light restriction reduces many aberration errors by preventing them passing the restriction. The darkfield loupe provides correct darkfield lighting for identifying small inclusions in gemstones and direction specific illumination on fracture fill inclusions. This is a small pocket portable, torch based instrument that has power full application in the diamond market of Europe, USA and Asia.
Recommendations
- Use your 10x for it can increase your knowledge and understanding of gemstones.
- A 10x lens is so much faster to use than a microscope that requires more detailed observation. Wear it out as soon as possible and buy a better quality lens as a Christmas present to yourself. If an excuse is needed, it will improve your observing and self-confidence. Do no think that the old 10x will do for the time being.
- Your 10x lens will repay with valuable information.
- Understanding information from a 10x depends on experience gained from informed and correct use f your most important instrument.
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