An insight and personal view of things gemmological.........

There is nothing more important to a gemmologist that his or her gemmological testing equipment. This may mean a lot of expensive and cumbersome equipment. For others it may mean smaller more transportable instruments that will virtually fit into a pocket. Whatever the number of instruments a gemmologist may have it is important to be aware of how they work and how they can be used. Here are some of the most often used gemmological testing equipment and some of the most interesting and bizarre.

10x loupe with achromatic and aplanatic lens

Appraisal EquipmentTo a gemmologist their eyesight is the most important possession they have after this a magnifying lens or loupe as it is referred to. I carry a Zeiss 10x loupe as pictured above. I was handed it in 1995 by my then employer H. L. Brown & Son Ltd, I was asked to sign for it. On leaving H. L. Brown some years later they kindly allowed me to keep it and I have it to this day. It is my most treasured piece of equipment and a symbol of when my enthusiasm in gemmology began. The Zeiss loupe is regarded as the finest lens available and is the standard against which all other loupes are judged. It is a folding pocket loupe specially designed for gemmologists and manufactured by the world renowned Carl Zeiss. Each lens is "aplanatic" (corrected for spherical aberrations which interfere with sharp focusing) and "achromatic" (corrected for chromatic dispersion, which causes colours to separate and show up as "halos" around bright details). They can be used for many things including grading colour, clarity and purity in diamonds and exceed every requirement for sharp, dispersion-free viewing. The tough reinforced polymer shell is much lighter than a metal frame, but provides better shock protection for the lenses in the event the loupe is dropped. Whether your inspection requirements are fine gems, jewellery, hallmarks, coins or even minute circuitry and biological specimens a Zeiss loupe will provide you with the sharpest image.

 

Binocular Microscope with zoom lens

Appraisal ExaminationThese microscopes are great for research and identification of difficult gemstones such as some synthetics but are not regarded as the be all and end all. Any gemmologist worth their salt would not normally rely on one of these big cumbersome pieces of equipment for identification. They are difficult, if not impossible, to carry and you will almost never find a plug when you need one. A 10x loupe is best used to grade and identify most gemstones. If you go to the New York Diamond Dealers Club you will not see row upon row of big expensive microscopes lined up on the tables. Because they don't need them! The main reason the keen gemmologist has a good microscope is to allow them to observe and appreciate the internal phenomena of a gemstone. It can be a fascinating world within the gemstone and to me the most interesting aspect of gemmology.  The following pictures represent what you could find when viewing a gemstone through a good microscope.

 

Gemmology StudyGemmologists AppraisalProfessional GemologyGemology AppraisalGemmological AppraisalGemological Appraisal 

Amber                                             Diamond                           Ruby                                 Quartz                        Fire Opal                                     Demantoid Garnet 

 

 

Refractometer.

Gemology EquipmentA refractometer is a device for the measurement of an index of refraction. The index of refraction is calculated from Snells law and can be calculated from the composition of the material using the Gladstone-Dale relation.

 

When I did physics at school the previous two sentence explanation of a refractometer is a good example of why I found the subject complicated and uninteresting hence the reason I was not successful in examination!

 

Put very basically a refractometer measures the extent to which light is bent (i.e. refracted) when it moves from air into a medium (gemstone in this case) and is used to determine the refractive index. The light bends because its velocity is reduced. The refractive index is expressed as a ratio of the speed of light in air relative to that in the considered medium (gemstone). In effect a refractometer calculates and displays the refractive index by dividing the speed of light in air by the speed of light in the medium. If you dip a straight stick into a pool of water at most angles it optically appears to bend. This is because light travels 1.33 times faster in air than it does in water; hence the refractive index of water is 1.33. A sapphire has an RI of around 1.77 meaning light is 1.77 time faster in air.

 

Gemmological ExpertiseA refractometer will measure most coloured gemstones having a refractive index up to 1.79. Some gemstones display double refraction (birefringence) this is when a single beam of light splits in two when passing through the gemstone in these cases the refractometer will display two results. For example a sapphire may show a result of 1.762 and 1.770 meaning its double refraction is 0.008.

 

Gemological ExpertiseMonochromatic light (1) enters through the rear of the refractometer through an opening (1a). It then hits a mirror (2) which transmits the light to the center of the hemicylinder (3).
This hemicylinder is made of high refractive glass (usually with a refractive index of 1.88 and a hardness of about 6.5 on Moh's scale).
At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder. The reflected rays (5) will pass through a reading scale (6) and a lens (7) or a series of lenses, depending on the type of refractometer.
The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside the refractometer to your eye (11).
The ocular (9) can slide in and out for better focus and is usually accompanied with a detachable polarizing filter (10).

 

Spectroscope

Advanced GemologyA small hand held tubular piece of equipment that a gemmologist should never be without. They are also used in astronomy to study visible light which radiates from distant stars and galaxies determining not only chemical composition but motion. In gemmology it is used to analyse light passing through a stone. When looking through a spectroscope you will see the colours red, orange, yellow, green, blue, indigo and violet. This is basically sunlight, white light as it is referred, that has passed through a prism and divided into its component 'rainbow' colours. Do you remember the old "spinning tops'. If you were to colour them with the seven colours of the rainbow in equal sections and set them spinning the result will be a visibly white spinning top. Colours are wavelengths (speeds of light) and measured in nanometres. The colour blue has a wave length of around 450 nanometres, orange around 650 nanometres. The retina of the human eye can only detect wavelength speeds of between 400 nanometres (shorter wavelengths) and 700 nanometres (longer wavelengths). This is called the visible light region of the electromagnetic spectrum. Beyond the visible range of the spectrum, undetectable to the retina, are ultra violet light and X-ray wavelengths which are much shorter wavelengths, below 400 nanometres, and infra red, microwave and radio wavelengths above 700 nanometres.

Gemological Results

When 'white light' passes through a stone, one or more of the wavelengths that produce colour are absorbed by the gem. The colours that are not absorbed are the colours seen when we look at the stone. The spectroscope is a tool for examining which parts of white light (within the visible range 400nm - 700nm) are absorbed by a gemstone.  When a gemstone is observed with a spectroscope, the absorbed parts show as dark lines and or bands in the spectroscope image. Each stone has a unique absorption spectrum it is like a gemstones 'finger print'. Here are a couple of crude examples of what might be viewed through a spectroscope. First a red medium where the absorption is of all wavelengths except red and secondly a purple medium where the absorption occurs of all wavelengths except blue and red. (blue and red combine to make purple)

Gemological Appraisal       Appraisal Results

Here are some typical examples of real absorption spectra.  In order:-                                                                                        Emerald                 Zircon                    Sapphire                 Ruby 

Appraisal Expertise    Expertise in Appraisal    Valuer Expertise    Gemmological Valuer       

 

 The Presidum Gem Tester

Advanced Gemmological EquipmentAny gemmological purist may well be offended that I have placed this piece of equipment under a section that has the word 'Gemmology' in its title. I apologise to those gemmologists, however, this is a very useful device particularly for valuers. It is a light and portable instrument provides a quick, easy, laboratory proven, way to identify diamond and separate the most popular coloured gemstones based on their thermal properties.

 

This tester is a thermal conductivity tester based on the fact that different gemstones conduct heat at different rates. It has a sophisticated thermoelectric probe, shaped like a pen which is used to probe the surface of any mounted or loose gemstone; the specially calibrated dial will show the relative heat conductivity of the material.

Advanced Gemology It does not differentiate between natural and synthetic gem stones which is something you could also say about refractometers. Its accuracy may be determined by temperature, state of the unit's batteries, recent measurement history and condition of the probe.

 

I was introduced to the instrument about five years ago when I visited the valuation department at Birmingham Assay Office. A very well equipped place with millions of pounds worth of testing equipment including spectrophotometer systems, liquid nitrogen immersion and photoluminescence equipment. They seemed quite proud and pleased to be using the comparatively basic Presidum Gem Tester costing less than £200.00! I thought if it was good enough for Birmingham Assay Office then it is good enough for me and purchased one shortly after.

 

Using such an instrument can/should only provide an indication of what a stone might be. For more comprehensive assessment of a gemstone this device should be used in conjunction with other, high precision, testing devices. Used like this it is a good instrument to use.

 

Ultra Violet Light

Simply put, ultraviolet light (also known as UV radiation and often referred to as 'black light') is a form of energy travelling through space.

 

Some of the most frequently recognized types of energy are heat and light. Heat keeps us from being cold, light enables us to see. These, along with others, make up what is known as 'The Electromagnetic Spectrum'. Only a small part of this spectrum can be seen by human eye sight and is referred to as 'visible light'.

 

The progression of electromagnetic radiation through space can be visualized in different ways. Some experiments suggest that these rays travel in the form of waves. A physicist can actually measure the length of those waves (simply called their wavelength ). It turns out that a smaller wavelength means more energy.  

Radio Waves       -     Microwaves     -     Infrared     -    Visible Light  -  Ultraviolet-X-Rays-Gamma Rays

 

Ultraviolet radiation is more energetic than visible radiation because it has a shorter wavelength. These waves are invisible to the human eye, though some insects, like bumble bees can see them! However it is not as energetic as x-rays which have even shorter wavelengths. UV is part of the electromagnetic spectrum which falls between 400nm and 10nm. Visible radiation includes wavelengths between 400 and 780 nanometers.

 

In gemmology testing two types of ultra violet light are used.  These are;
· Long wave ultra violet (LWUV) having a principal wavelength of 365nm
· Short wave ultra violet (SWUV) having a principle wavelength of 254nm

 

When high energy rays in the form of invisible ultraviolet light are beamed on various substances, they glow with visible light. This phenomenon is called 'fluorescence'. The colouring elements in a number of substances are influenced by the high energy of the ultraviolet light rays. The state of their electrons is altered from normal to excited. The electrons immediately return to their 'normal' state, and in doing so release low energy in the form of visible light. Thus, certain stones placed in a dark container and irradiated by supposedly invisible ultraviolet light fluoresce with a specific colour. This colour can be a completely different colour to the stones 'real' colour as seen under normal light. In some cases, the fluorescent effect continues after the ultraviolet light radiation has ceased. This, afterglow, phenomenon is called phosphorescence. This effect is caused by a delay in the return of the electrons to their 'normal' state.

 

Modern gemmology makes use of this phenomenon. In some cases, the nature of the stone's reactions to various radiations can aid its identification.

 

The ultraviolet light illumination device used in gemmology is made up of a box which is opaque to light and fitted with an observation point. Installed above the box is a special lamp capable of producing long wave and short wave ultraviolet rays. Fluorescence under short wave light is generally weaker.

 

Diamonds may fluoresce strongly, weakly or not at all. They can produce many colours but mainly bluish, whitish, greenish and even pink or yellow. If you were to examine a cluster or group of diamonds under long wave ultraviolet light you would expect to see a number of different results. If you observe the same result in all the stones there is an extremely good chance you are not observing diamonds!

Similarly with many coloured gemstones you can observe many differing results which can mean many things. Sometimes the effects of ultraviolet light on a gemstone can indicate where it comes from

 

 

Geiger Counter

Bizzare Gemological EquipmentA somewhat bizarre addition to a collection of gemmological instruments is the Geiger Counter. What on earth has it got to do with gemmology you may well be thinking? Well yes there are radioactive gemstones out there!

 

Before I go any further I must explain. Please do not be scared and don't let this put you off gemstones. The sorts of radiation levels we are talking about are insignificant. There are household items like pottery, some metal furniture and camera lenses that also contain similar insignificant amounts of radiation. In fact mobile phones probably register higher levels and despite extensive research on the subject, there has been no conclusive evidence that using a mobile phone causes long term harmful effects in humans.

 

A Geiger counter is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. The model in the picture is a Victoreen CDV700 dating back to the early 1960's and originally used during the 'Cold War'. This will read lower radiation levels that gemstones will offer. Other models may not read such low levels.

 

Certain gems are artificially irradiated by the radioactive element Radium, X-rays, or other particles in order to enhance its optical properties. High levels of ionizing radiation can change the atomic structure of the gemstone's crystal lattice, which in turn alters the optical properties within it. As a result, the gemstone's colour may be significantly altered or the visibility of its inclusions may be lessened. The process, widely practised in the jewellery industry, is done in either a nuclear reactor for neutron bombardment, a particle accelerator for electron bombardment, or a gamma ray facility using the radioactive isotope cobalt-60. Irradiation enables the creation of certain gemstone colours that do not exist or are extremely rare in nature. It can make the gem itself radioactive which can last for years in some cases.

 

The most commonly irradiated gemstone is topaz which becomes blue after the process. Blue topaz is very rare in nature. Approximately 30 Million carats (6000kg) each year are irradiated globally. Quartz too may be irradiated to produce amethyst. These are examples of artificially irradiated gemstones. Natural gems like blue and green diamonds are the result of the exposure to natural radiation during there formation in the earth.

 

Irradiation is just one of many gemstone treatments found within the jewellery trade. The vast majority of gemstones you will see on sale will have received a 'human' induced treatment or enhancement, like heating, oiling, dying or laser to improve its appearance. Were it not for these treatments some gemstones would not be as beautiful and certainly less attractive.

 

As a footnote there are other things in the jewellery trade which will register on a Geiger Counter. Cloisonné, an enamelled variety of jewellery, is glazed with Uranium oxide. Watches, some old watches and clocks, even gyroscopes, have dials painted in radium to make them glow in the dark.  Radium emits Alpha and Gamma rays.  More recently, Tritium, a radioactive isotope of Hydrogen, has been used to paint the dials, but it is too weak to penetrate the watch lens.

 

Invented in 1908, Geiger counters remain popular instruments used for measurements in health, physics, industry, geology and other fields, because they can be made with simple electronic circuits. I would suggest any gemmologist possessing one really is fanatical. It certainly is an interesting piece of equipment!