A lot about electronic gem testers


Several models reviewed







Electronic gem testers
Optical gem testers

NOTE: in the jewellery trade, "Diamonds" and "Gemstones" are regarded as separate industries. Diamond testers (see separate article) are more precise since they are fine-tuned for testing diamonds only.


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 gives an approximate selection of stones, or something more complicated and time-consuming that gives a more precise selection.

In addition, there is no 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.

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 "100% guarantee" of accuracy, let us know where you have seen it and we will give you our opinion.

Examples of when gem testers are useful:

- you have been given a parcel of stones and want to pull out those that are 'interesting' 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 imitation - what is it likely to 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.

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

    Special note about diamond and Moissanite

    All 'simple' diamond testers will register the manmade stone Moissanite as being diamond. "Diamond/Moissanite" diamond testers (also called 'Multi' diamond testers) will distinguish the two. The most popular model is the Multi Experior, and it will also indicate ruby/sapphire (but no other gemstone).

    If you are after a general gem tester (as listed on this page) that can also distinguish diamond from Moissanite - the thermal type cannot do this. Go for the reflectivity/Refractive Index type instead. 

    The advantage of using a reflectivity/Refractive Index tester is that you can use it for many gemstones including diamond and Moissanite. The disadvantage is that it is a sophisticated piece of laboratory equipment which needs to be used with great care. By contrast, a dedicated Diamond/Moissanite (or 'multi') tester is rugged and easy to use.

    Conclusion: if you want to test diamonds/Moissanite in a busy shop or at markets and fairs, get a Diamond/Moissanite ('multi') tester, they are very easy to use, especially if you are working under pressure. If you are working in an office where you can sit comfortably at a desk and work slowly and carefully, then get a reflectivity/Refractive Index gem tester as listed below. If you manage a group of stores (or an auction house) that does both - then get both.


    Thermal 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.

    Thermal testers: detail

    - works on any stone, you are not limited to jewellery 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.
    - 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
    - if you need something that is reasonably rugged, not a 'sensitive item of laboratory equipment'
    - if you are only interested in being able to test for the most common gemstones
    - these are good for showing what a stone is NOT, e.g. it might be glass. it's unlikely to be quartz, it 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 must be used at room temperature.
    - will not distinguish diamond from Moissanite 

    Reflectivity / Refractive Index testers are optical not thermal (so they work at any temperature), they help identify many dozens of stones.

    Advantages of reflectivity / Refractive Index testers:

    - if you really do need to be able to test 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 perfectly flat, perfectly polished top (table) which will fit over the sensor hole; if the stone is tiny (under 3mm) you will struggle to get a reading; if an item of jewellery is too large (or lumpy) you won't be able to rest the stone over the hole
    - if you are quite happy to work slowly and carefully in laboratory-clean conditions but you must have a machine that will positively identify any stone: such a tester does not exist
    - if you might be testing large items of jewellery (e.g. bracelets, bangles, necklaces), they won't fit over the tiny sensor (they will fall off)
    - if you understand the limitations of gem testers but there really is no way you have time to keep the tester meticulously clean and clean every stone before every test, and you cannot guarantee that it will never be bumped or dropped or get dusty or damp.

    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.


    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:

    Presidium Handheld gem tester - buy now (thermal conductivity)

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

    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
    - the first gemtester 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!) -


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


    Jade (Nephrite)

    Jade (Jadite)


    Tourmaline group: Paraiba, Rubellite, Tourmaline

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

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




    Corundum group: Ruby, Sapphire



    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 (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 with the PRIM II Chart. 

    Digital refractometer (reflectivity meter)

    The reflectivity meters (above) display 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 stonesYAG, 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.