Electronic gem testers

Last updated: June 28, 2024

GEM TESTERS

Several models reviewed

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ELECTRONIC GEM TESTERS

This article is quite long, it is not necessary to read all of it, click on the sections in the CONTENTS that interest you. Alternatively, skip this article (with all its advice and recommendations) and go to our Diamond and gem tester comparison chart which simply lists each model and what it can detect.

See also our guide to buying diamond testers. You might also be interested in specifically testing synthetic diamonds or maybe testing for Moissanite.

PRODUCTS
INTRODUCTION
EXAMPLES OF WHEN TO USE AN ELECTRONIC GEM TESTER, AND WHEN NOT TO USE ONE
THERE ARE TWO TYPES OF ELECTRONIC GEM TESTER
ELECTRONIC GEM TESTERS - WHICH MODEL TO BUY?
RELATED ARTICLES

PRODUCTS

Electronic gem testers
Optical gem testers

INTRODUCTION

There is no gem testing machine that will simply light up with the name of one gemstone, giving a positive 100% guaranteed identification. Each time you test a stone, the reading will give a selection of possible stones.

When choosing a model, the choice is between something very quick and easy to use that tests for a small selection stones, or something more complicated and time-consuming that tests for a larger selection of stones No matter what the result, it is always possible that you have any one of a thousand other stones which the tester does not test for.

Gem testers merely suggest possibilities and you must use a combination of experience and other tests to come to a conclusion. If you see a gem tester (any gem tester) elsewhere with a "guarantee" of accuracy, let us know where you have seen it and we will give you our opinion.

Apart from diamond, there is no electronic machine that will distinguish natural gemstones (grown in the ground by nature) from synthetic stones (grown in a laboratory using the same chemicals and the same conditions found in nature). "Synthetic" is not to be confused with "imitation" (or "simulant") where one stone 'imitates' another, e.g. blue spinel might look like sapphire, but it isn't - it's spinel; Moissanite might look like diamond, but it isn't - it's Moissanite.

EXAMPLES OF WHEN TO USE AN ELECTRONIC GEM TESTER, AND WHEN NOT TO USE ONE

YES! If..

you have been given a parcel of stones and want to pull out those that are 'interesting' (e.g. aren't glass) and worthy of further investigation.
your supplier of new jewellery has sent you ten sapphire rings and you want to check that they are all the same.
you have been offered an antique ring described as aquamarine, how likely is this?
you are in dispute regarding a stone that you know for certain is not diamond, it looks like diamond but it is an simulant (imitation) - what might it be?
you know, from your experience, that a customer is offering you paste, he says it's white sapphire, you need to show him the result on a machine, because customers believe machines rather than expert humans.

NO! If...

you need a tester that can identify any stone you are ever likely to come across
you don't mind if the tester only tests for a few stones but it must be 'guaranteed' that it is that stone
you understand the above limitations, but you really can't cope with only being given a selection of possible stones.

Too complicated? Then do not buy a gem tester (or if you do, please do not buy it from us). Sounds interesting? - then read on.

THERE ARE TWO TYPES OF ELECTRONIC GEM TESTER

Thermal conductivity testers work on heat. Their advantage is simplicity, touch the probe on the stone, see the reading, no skill and no special training is required. Disadvantages: they only work at room temperature (typically between 19 and 26 degrees C); they only test for a few stones and (as you see from the picture below) many of the readings overlap.

Advantages of thermal testers:

works on any stone, you are not limited to perfectly-polished loose stones that will fit over the sensor, you are not limited to transparent stones with a flat top (table), they even work on 'rough' (uncut) stones.
good if you (or your staff) really don't have time for 'learning' or 'training', or for working slowly and carefully, you just want something quick and easy to use
good if you need something that is reasonably rugged, not a 'sensitive item of laboratory equipment'
good if you are only interested in being able to test for the most common gemstones
good for showing what a stone is NOT, e.g. thta it might be glass, is unlikely to be quartz and can't possibly be sapphire.

Disadvantages of thermal testers:

they test a limited number of different stones and there is a lot of overlap (see picture above)
not ideal for buying at outdoor venues when it's cold, the tester (and the stone) must be at room temperature.
will not distinguish diamond from Moissanite

Refractive Index (RI) testers (technically, these are 'reflectivity meters')

Advantages of reflectivity / Refractive Index testers:

good if you really do need to test for as many gemstones as possible
if you also need to distinguish diamond from Moissanite
if you want to identify diamond simulants (including Moissanite)

Disadvantages reflectivity / Refractive Index testers:

the stone must be transparent and must have a perfectly flat, perfectly polished top (table), and large enough to fit over the sensor hole (3mm)
these are very sensitive 'laboratory' testers, you must keep the tester meticulously clean and clean every stone before every test, you must not drop it or bump it, or get it dusty or wet.
as with all testers (and even if you work slowly and carefully in laboratory-clean conditions) the result will be a selection of stones, and you must use your expertise and experience to decide which one it is

Comments from experts about Presidium reflectivity / Refractive Index testers:

Here are some independent evaluations from two internet forums run by gemmologists. Gemmologists are scientists, they are not interested in sales hype:

Just this morning some students and me have been checking the Presidium Duo's reflectivity meter. In this case we tried to find out if a stone purchased on internet was indeed a GGG or a CZ. This reflectivity meter worked (in this case). The result was proven by its SG.
It is good for quick sampling, but you do need to keep it calibrated with known samples. I also haven't been able to get it to work reliably with tiny stones. Relying on any one characteristic to identify a material practically guarantees misidentification.
Used in combination with other tests I find this tool quite vital and use it daily
I use it for doing those quick IDs that have two possibilities and need a quick decision on which one it is, works perfectly
An approximate reading obtained quickly and neatly is very useful in confirming ID in many parcels. Spinel parcels often need the garnet weeded out, Tsavorite parcels need the chrome tourmaline weeded out, and so on. Seems like this may be a time saving instrument on checking parcels of many varieties.
Used in combination with other tests I find this tool quite vital and use it daily
I use it for doing those quick IDs that have two possibilities and need a quick decision on which one it is, works perfectly
An approximate reading obtained quickly and neatly is very useful in confirming ID in many parcels. Spinel parcels often need the garnet weeded out, Tsavorite parcels need the chrome tourmaline weeded out, and so on. Seems like this may be a time saving instrument on checking parcels of many varieties.

ELECTRONIC GEM TESTERS - WHICH MODEL TO BUY?

The 'simplest' thermal tester has an analogue meter:

Standard Presidium - buy now (thermal conductivity)

See above for the advantages and disadvantages of thermal testers. This is a thermal tester.

You can see from the picture which stones it will test, also note how the readings overlap. It is for this reason that I say the tester is best for showing what a stone is NOT rather than what it is, e.g. a customer is offering you a sapphire, you can see that it might be glass, it might be garnet...but it can't possibly be sapphire. Despite its limitations, it is still useful for pulling out gemstones worthy of further investigation, e.g. you may be interested in stones that might be ruby/sapphire and not at all interested in anything that is probably glass. It is also particularly easy and fast to use and can be used on any stone no matter how it might be mounted.

The most sophisticated thermal tester displays the names of the stones:

PGI Presidium Handheld gem tester - buy now (thermal conductivity)

See above for the advantages and disadvantages of thermal testers. This is a thermal tester.

The PGI is a more sensitive version of the Standard Presidium, above; and the design has been 'squished' to the size of a standard diamond tester.

Advantages of this particular model:
nearly as easy as the 'Standard' Presidium, above - it has more settings, you can narrow the results by selecting the colour of the stone you are testing
it's the first gem tester that is truly pocket-size, with the finest probe tip, will test stones down to 2pts in size.
replaceable probe (i.e. if you break it)
latest technology, this is the only tester of this sophistication that is pen-size, operated with one hand, slips into a pouch (supplied)
super-fast warm-up time between tests (3 seconds), auto-power-off (if not used for 10mns)
takes standard batteries; can also be powered from a USB socket on a mains plug (supplied) or on a computer (not supplied)
will give an indication on 31 different gemstones

Disadvantages of this particular model:

it is an extraordinarily sensitive thermal conductivity meter, and is therefore very sensitive to heat. If you are not using it at room temperature you must calibrate it (not difficult but it does take several seconds); it really will not work at all if it's very cold (e.g. outside in the winter in Northern Europe)
there is a large amount of overlap in the readings (click on the picture):

Here is its full list of gemstones the Presidium handheld gem tester will detect (but please study the chart above, so see how the readings overlap!) -

Glass

Beryl group: Aquamarine, Emerald, Goshenite, Helidor, Morganite.

Tanzanite.

Jade (Nephrite)

Jade (Jadite)

Peridot

Tourmaline group: Paraiba, Rubellite, Tourmaline

Garnet (Grossular-Andralite group): Demantoid garnet, Hessonite, Tsavorite

Quartz group: Chrysophrase, Amethyst, Adventurine, Citrine, Quartz

Spinel

Chrysoberyl

Topaz

Corundum group: Ruby, Sapphire

Diamond.

There is one model of thermal meter that also has a reflectivity meter, the Duo Presidium:

Duo Presidium (thermal conductivity and reflectivity)

This has both an analogue thermal meter operated by a probe and a digital reflectivity meter (for reflectivity/refractive index).

The analogue meter is very easy to use (on both loose and on set stones) and tests 17 stones, most of which have overlapping readings (see chart below). To see all the advantages and disadvantages of the analogue (thermal conductivity) meter scroll up to the 'standard' Presidium model.

The digital RI (reflectivity) meter will test 40 stones but it will only test loose stones (not stones set in jewellery) and the top (table) of the stone must be absolutely clean, perfectly flat, and perfectly polished. If there are any scratches or patches that aren't perfectly polished (use a good quality loupe!) take 4 or 5 readings at different spots of the table and reference the highest result on the PRIM II Chart provided.

Digital refractometer (reflectivity meter)

Whereas the reflectivity meter on the Duo Presidium (above) displays the result as a Presidium number you must look up on a chart; this reflectivity meter displays the reading as refractive index. The advantage of this format is that it's a standard system all gemmologists understand it, all gemstone books and websites list the same numbers, you will soon get to recognise many of them.

Electronic refractometer (reflectivity meter), advantages:

pinhole-sized sensor for testing stones down to 3mm diameter
instant reading and no waiting time between tests
pull-out refractive index chart as a quick-reference for diamond / diamond simulants (including Moissanite) and
pocket-sized Refractive Index Chart with R.I. readings for 55 stones
built-in 8-piece gemstone holder (sets of sample gems are an optional extra)
battery-operated plus USB port for alternative power source and (if required) to connect it to a computer.
use the downloadable software (from Presidium), each test will display data on the computer listing many variables (in addition to refractive index) that will help you identify the stone.

Electronic refractometer (reflectivity meter), disadvantages:

the surface of the stone must be perfectly flat, perfectly polished and perfectly clean. If there are any scratches or patches that aren't perfectly polished (use a good quality loupe!) take 4 or 5 readings at different spots of the table and use the highest.
it is a sensitive piece of laboratory equipment, it must be kept clean and dry, the sensor must be kept dust-free, it must not be bumped in and out of bags or used on damp outside market stalls.
as with all testers, there are many stones with the same, or similar, readings.

Compared with an optical refractometer, advantages:

gives higher readings (1.00 to 3.00) and this enables it to test for diamond and for simulants such as Moissanite
no contact fluid required
fast and easy to use, easy-to-read digital display

Compared with an optical refractometer, disadvantages:

it's not as versatile as an optical refractometer, you only get one reading for each stone (technical: you can't measure the birefringence). This means that if you want to distinguish diamond simulants you need to compare actual stones (get a sample set of stones: YAG, GGG, synthetic sapphire, synthetic spinel, zircon, strontium titanate and cubic zirconia)
not as accurate as an optical refractometer, it's possible (as with all electronic testers) for the display to 'drift' and not give correct readings.

See also our guide to buying diamond testers. You might also be interested in specifically testing synthetic diamonds or maybe testing for Moissanite.