Instruments Used By Gemmologists
Have you ever wondered what tools are used by a gemmologist to identify gemstones? Most fit in your pocket and help you to identify the gemstones. But are they good enough to determine if the gemstone is synthetic or has any treatments?
Tools For a Beginner in Gemology
The first items to purchase would be a loupe, light and tweezers. Lighting is very important and you will find that some lights are better with different instruments. I like the mini-maglite (non-LED) as an all-purpose light.
Gemmologists also use fibre-optic lights, LED, long-wave and short-wave ultra-violet lights for various tests.
I place a rubber band around all my cylindrical instruments so they do not roll off the table!
The tweezers should have some sort of grip at the tips to hold the gemstones. Locking tweezers are good for holding a stone in a position that can be viewed later or passed to someone else to examine. When using locking tweezers be careful not to squeeze the gemstone too hard, this can cause damage to the gemstone or cause it to slip out of the grip and fly out of the tweezers. A fine tip would only be used for very small gemstones.
A loupe should be 10 power (10x), meaning that the gemstone is magnified 10 times its actual size. Diamonds are graded at 10 power magnification. The higher the magnification the shorter the focal length, if you have higher than 10x then it will be difficult to use as it will have a working distance of only 1.6cm and a very shallow depth of focus. An 18mm size of the lens is sufficient. If you purchase a larger sized loupe the area that is in focus does not become larger.
What would I look for with a 10x loupe?
Surface damage – e.g. chipping, scratches, surface flaws
Quality of the cut – e.g. symmetry
Quality of the finish – e.g. polishing lines
Sharpness of facet junctions
Inclusions – e.g. crystals, silk, veils, bubbles
Colour distribution and growth zones
Curved growth lines in synthetic materials
Straight colour zoning in natural gemstones
Strong double refraction
Composite stones – e.g. a change in lustre, junction planes, bubbles, dust or cement lines
Instruments Used by Professional Gemologists
Trained gemmologists will use many of the following instruments listed. I have given at least one practical gemstone example for each instrument. All gemstones can be tested with all these instruments, each giving predictable results.
Dichroscope – This instrument can be used to differentiate red glass from ruby or garnet from ruby. Light is transmitted through the gemstone and the observer looks for one or two colours in squares (in a calcite dichroscope) or two different colours side by side (in a London Dichroscope).
Spectroscope – Sapphire, emerald, ruby and many other gemstones can be identified with this instrument. Light is transmitted through or reflected off of a gemstone and the absorption spectrum displayed is noted and compared to existing absorption spectrums. This is a diagnostic instrument and is able to identify a gemstone that produces an absorption spectrum.
Ultra-violent Lamp – A ruby can be differentiated from a garnet. Short-wave and long-wave radiation is used to test the gemstone’s reaction to UV radiation.
Colour Chelsea Filter – Blue sapphire and other blue stones coloured by cobalt can be differentiated from each other with the CCF. By shining light through or onto a gemstone different elements contained in gemstones will react differently to white light when viewed through the filter. There are other filters used by gemmologists for specific gemstones.
Specific Gravity Measurement – A cubic zirconia is much heavier than a diamond of the same size. A gemmologist can measure the relative density (specific gravity) of a gemstone using specific gravity liquids, direct measurement (Hanneman Balance), a water displacement method, or a one-pan or two-pan balance.
Immersion Cell – This container holds a liquid and I will put the gemstone into the liquid. If I have a liquid that bends (refracts) light at a similar amount to the gemstone being tested, then reflected and refracted light rays will be removed and I will be able to see the inclusions more clearly. I can also use liquids of a known refractive index to estimate the refractive index of a gemstone. Refractive index is the amount that the light rays slow and bend when they enter a gemstone or liquid.
Polariscope – A polariscope will differentiate between blue spinel and sapphire. With the use of the conoscope (a strain-free glass ball) we can differentiate between blue topaz and aquamarine. The polariscope can tell how light rays react when they enter a gemstone. With the help of a conoscope we can get even more information about the light rays in the gemstones.
Refractometer – The refractometer (using a refractive index liquid) will give readings that enable the gemmologist to identify most gemstones. With the use of a chart of known refractive index readings we can identify most gemstones.
Is the Refractometer the Only Instrument Needed in Gemmology?
Some people think that if a person can use the refractometer and interpret the results then this is the only instrument that they need. It is certainly an instrument that is diagnostic and can identify most gemstones. But gemstones that cannot have a good contact with the glass prism on the refractometer will not give a reading, perhaps their facets are too rough or scratched to make good contact. If a gemstone is set in jewellery and the setting prevents the gemstone from resting with a surface that touches the glass prism on the refractometer, then a reading cannot be obtained. This is a common problem with gemstones set with large claws or prongs. Some gemstones have readings that are out of the range of a refractometer, certain garnets will not have a reading on a refractometer. A diamond is too hard and would scratch the glass prism and should never be used on a refractometer, it has a reading over the limit and would not give a reading anyway.
A refractometer will not distinguish between natural and synthetic gem materials and will not indicate if the gemstone has been treated. Sometimes a combination of the refractometer and another small instrument will help you to determine if the gemstone is a true synthetic.
The Gemmological Microscope
A gemmological microscope is a binocular microscope that has a zoom magnification power of about 10x to 90x. A good gemological microscope should be equipped with light-field and dark-field sub-stage illumination, overhead illumination and preferably an adjustable iris. This is the instrument that will help you determine whether a gemstone is natural or synthetic. The microscope is also used to look for treatments.
How to use a Gemological Loupe?
If you are thinking about becoming a gemmologist remember that being able to use the instruments is not good enough. You need to be able to interpret the results correctly!
I have a gemstone that shows acicular inclusions in the microscope, shows orange-red and red through the dichroscope, shows a chromium spectrum through the spectroscope and it has a reading of 1.760 to 1.768 on the refractometer. What gemstone is it? It must be a ruby!
(Join our poll or Scroll Down To The Bottom and leave a comment) – Emerald is winning the poll!
I plan to introduce different gemstones alphabetically. I will start with “A” (Ammolite) and see where it leads me. Leave a comment and let me know if you have a favourite gemstone that starts with the letter “L”. Take the poll! Can you name a reference to a gemstone in a song, movie or book?