A little about optical gem testers
OPTICAL GEM TESTERS
Spectroscope, Chelsea Filter, refractometer, polariscope
We also sell electronic gem testers.
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CONTENTS
SPECTROSCOPE
CHELSEA FILTER
POLARISCOPE - HANDHELD & MAINS-POWERED
HARDNESS PENCILS
RELATED ARTICLES
SPECTROSCOPE
SUMMARY
Diffraction grating spectroscope, a tube 50mm long X 12mm dia.
You view the light from coloured stones, you see a spectrum of colour (rainbow) interrupted with black (or white) lines.
This is because some elements (transitional elements) absorb specific wavelengths (colours) of light, appearing as black lines; some re- transmit specific wavelengths, appearing as white lines. These patterns of lines give clues as the chemical composition of the stone.
You don’t have any gemstones to hand? Point it at the sky and see the lines that correspond to hydrogen burning in the sun and oxygen being absorbed by the atmosphere. Look at the dozens of lines caused by the gases burning in a fluorescent light tube. Shine a torch through your finger, see the pattern of lines caused by iron in your blood.
WHAT DOES IT DETECT?
Spectra, patterns in a spectrum (rainbow). Note: you are looking for distinctive patterns of light and dark bands, you are not taking measurements, this is not a spectrometer as used in physics and chemistry to measure specific wavelengths.
IS IT EASY TO USE?
If you like colours and are good at recognising patterns of lines, you’ll love this tester. Get to recognise the patterns of lines that indicate chromium and iron, instantly identify synthetic stones (and glass) which are bright cobalt-blue, see the myriad of lines in some zircon caused by radioactive decay.
This does not come with instructions, you will need to buy a good book about gemmology to see pictures of the patterns of lines and to find out about the chemical composition of gemstones. Or here is a website with dozens of really good illustrations (the links listed by colour, on the left, are in Excel format so you have to be patient while it downloads).
LIMITATIONS
If you are looking for a machine that will give a reading (a number) you can look up in a book, or if you want “guaranteed identification”, then this tester will not be for you.
CHELSEA FILTER
SUMMARY
The Chelsea Filter, as developed at the Chelsea College of Science and Technology in 1934. Our is the actual Chelsea Filter, not a copy (see the picture at the top of this page, this is what it should look like!).
Many stones appear to change colour when viewed through the filter. Originally designed for detecting synthetic emeralds, nowadays the Chelsea filter is an invaluable aid to identifying many other gemstones.
It can give an indication that you have an emeralds and aquamarines, though not a guarantee, since there is considerable variation in their chemical compositions (the amount of chromium that gives the green colour and the amount of iron that ‘dampens’ the effect through the filter).
It gives a very clear and spectacular result on blue stones coloured with cobalt. Since cobalt doesn't colour stones blue in nature, this is a sure-sign that you have a manmade stone (e.g. synthetic spinel) or glass.
With skill, it helps identify dozens of other stones. It’s particularly useful when you have to sort large parcels of stones into ‘probables’ – then you have clues as to which tests to carry out next.
WHAT DOES IT DETECT?
Apparent colour change in some stones.
IS IT EASY TO USE?
USING it is very easy indeed. Shine a light (any pocket torch) on the stone, hold the filter close to your eye, view any colour change. You don’t even have to hold the stone close to the filter (only the filter has to be close to your eye) so you can examine stones in museum showcases.
LIMITATIONS
There is an instruction sheet supplied with the filter. However, in common with all gem testers, it does not provide an instant, easy and guaranteed identification of gemstones (the only ‘easy’ one is detecting cobalt-blue stones, which are synthetic, or glass, but never natural).
FURTHER INFORMATION
Gemstones is an excellent book about gemmology, with lots of illustrations of stones.
WE have also written our own instructions.
SUMMARY
The refractometer is the most important gem tester after a loupe. It measure refractive index (how far light is bent as it passes through the gemstone).
There are two models, the basic model doesn’t have a light source. Well, it does, it comes with a pencil torch, it works fine, but it's ugly. The special built-in ‘monochromatic’ light (yellow,580-590nm) is neater.
Any serious book about gemmology has pages of charts of refractive indices (or try this website), along with the theory of how light refracts (bends) through different media. Armed with this knowledge and a refractometer you are well on your way to identifying many hundreds of gemstones.
This item is not supplied with instructions.
WHAT DOES IT DETECT?
Refractive index (RI).
IS IT EASY TO USE?
Moderately easy.
Put a tiny spot of contact fluid on the test-plate, rest the flat polished surface of the stone on fluid, and through the viewfinder you see a scale, part of which will be light and part of which will be dark. In between the light and dark is the ‘shadow edge’ that marks a reading on a scale. Using daylight, or a torch, the shadow edge will be fuzzy, you will be get an approximate reading. Using a monochromatic light (the better of the two models) you will see a sharp shadow edge and a more precise reading.
Not only do you get a reading, but often you will see two shadow edges, indicating a doubly refractive stone (which, itself, is diagnostic) and advanced users can glean further information by rotating the stone to see if either or both of the shadow edges move, and by taking minimum and maximum readings for each.
Getting a basic reading on the scale takes a little practice; for advanced users, seeing which way two shadow edges move as you rotate the stone takes a lot of practice.
LIMITATIONS
Although this is an ‘easy’ test because you end up with a number, and you look up the number on standard charts, you will soon discover that there are dozens of gemstones with the same (or overlapping) values. A refractometer will help you narrow the possibilities to a handful (maybe one or two) stones, but it will not give an instant 100% guaranteed answer.
This item is not supplied with instructions.
FURTHER INFORMATION
Gemstones is an excellent book about gemmology, with lots of illustrations of stones.
POLARISCOPE - HANDHELD & MAINS-POWERED
SUMMARY
Handheld: A tube 80mm tall X 20mm diameter, it detects double refraction.
Mains: 7 X 6 X 5 inches, and the filter (“platform”) 35mm diameter.
Most gemstones are crystalline in structure. Some crystal structures are doubly refractive and some are singly refractive. For instance, diamond is a crystal of the cubic system, it is singly refractive. Most diamond simulants (look-alikes, not diamond) are doubly refractive, including Moissanite. So you could use this as an inexpensive tester to distinguish diamond from Moissanite. You cannot, however, use it in place of a diamond tester, for instance, the significance of Cubic Zirconia is that it’s crystal system is cubic, just like diamond.
WHAT DOES IT DETECT?
Double refraction.
IS IT EASY TO USE?
Reasonably easy. At the top and bottom of the tube are two filters (Polaroid discs), revolve the top filter until everything in the tube appears dark and place the stone on the platform in the middle. Now turn the stone so that it revolves through 360°. If it appears to turn light-and-dark it is doubly refractive, if it remains dark it is singly refractive.
LIMITATIONS
The stone needs to be rotated on the platform. That’s very easy with loose stones, but there isn’t much room to fit stones mounted in jewellery in the handheld version, just 12mm diameter. The mains version has nice big (35mm) platform, but you do need mains power, it does not take batteries.
Light must be able to pass through the stone, not a problem with transparent stones (a torch is provided, it fits into the polariscope) but you cannot test opaque stones.
FURTHER INFORMATION
Gemstones is an excellent book about gemmology, with lots of illustrations of stones.
HARDNESS PENCILS
SUMMARY
Each ‘pencil’ has a tip of a different hardness from No.5 to No.9 on the Mohs scale of hardness (scroll down slightly for Further Information).
WHAT DOES IT DETECT?
Hardness
IS IT EASY TO USE?
Yes.
See which will scratch your sample and which will not.
Out in the field, use them for testing minerals, though do remember that hardness varies from sample to sample, and it can also be different in different directions.
LIMITATIONS
Out in the field, use them for testing minerals, no problem, but on cut gemstone the procedure is slightly different. You scratch the customer’s stone, the customer sues you. In other words, do not use this on cut gemstones. Many people ask us for these for use on cut gemstones, they say, “I’ll be careful”. OK, I can only give my advice: don’t use a scratch test on a cut gemstone.
FURTHER INFORMATION
The full Moe scale goes like this:
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond
Gemstones is an excellent book about gemmology, with lots of illustrations of stones.
To buy optical gem testers click here.