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The Presidium Gem Tester has been developed as an aid in distinguishing between diamond and its simulants. It makes us of the fact that diamond conducts better heat than any other material, and it is foreseen that no other substance, whether natural or synthetic, will ever be comparable with diamond in this respect. Other properties of diamond, such as its high refractive inex, can be closely matched by other materials and they are, therefore, less useful in identifying this most precious gemstone. The probe consists of two linked thermometers, one of which is heated electrically while the other is cooled by the gemstone being tested. The difference between the temperatures of the two thermometers is a measure of the heat conductivity of the gemstone and gives rise to an electrical output that is amplified and displayed on a meter. The meter reading thus indicates the heat conductivity.


1. The instrument can be operated either from mains voltage (through the adapter) or from batteries.

2. When the mains adapter is used, a check should be made that it is set correctly to the local voltage. Only the adapter supplied with the instrument should be employed.

3. When operating from a power point the single-pin plug of the adapter should be inserted in the socket behind the meter.

4. If batteries are to be used, the adapter should be disconnected from the meter unit and two size AA batteries should be inserted in the battery compartment underneath. Alkaline batteries should be used if possible since they will give about 4 hours of continuous operation. Ordinary batteries may be used but will give a shorter working life. Batteries do not have to be removed when the mains adapter is in use.

5. The probe should be linked to the meter unit by inserting the multi-pin plug at the end of the cable into the right-hand socket.

6. Turn on the instrument. Wait for about 20 seconds which is the initial warm-up period of the unit. A small reading on the meter, above or below zero, at this stage is quite normal.

7. Simulant test disc. Press the tip of the probe on the disc located to the left of the indicator lamp. Within two or three seconds the needle should reach its highest point well within the red band of the meter display.

8. Diamond test disc. Press the tip of the probe on to the disc located to the right of the indicator lamp. Use as much pressure as will make the protruding wire tip retract completely into the end of the probe pen. Within two or three seconds, the needle should rise and reach its highest point well within the green band of the meter display.

9. Adjusting the meter. The instrument has been completely calibrated during manu-frature and no further adjustment should be needed. If, however, adjustment does become necessary. Remove the protective cap over the hole at the rear of the meter unit indicated by the letter CAL. The recessed slotted knob can then be turned carefully with a screwdriver until the meter gives the correct readings on test.


After setting up the instrument according to the instructions on the previous page, it is now ready for use.

    1. Press the tip of the probe against the gemstone to be tested applying enough pressure to make the wire tip retract completely into the probe pen. Maintain a steady contact of the probe with the gem surface and hold the probe at a slight angle.
    2. Observe the highest reading on the meter. The needle will rise rapidly, reach its highest value within two or three seconds and then fall slowly.
    3. If the reading falls in the green band, a diamond is indicated. If it falls below this band a simulant is indicated.
    4. The Presidium Gem Tester can still be used when the batteries are nearing the end of their life provided that the indicator lamp still comes on. However, if it does not glow brightly an increase in the warm-up time must be expected and allowed for. .
    5. Metal Alert Buzzer. If the tip of the pen comes in contact with the metal mounting of a stone, an audible signal is emitted.


  • The gemstone should appear to be clean and dry before testing but elaborate cleaning procedures are unnecessary.
  • Rather low readings in the green band must be expected with very small diamonds.
  • It is advisable to take a number of measurements and from time to time use the test discs to check that the instrument is functioning correctly.
  • Low readings, near the bottom of the red band will be observed for glass and cubic zirconia and rather high readings, near the top of the red band, for sapphires and rubies.
  • The instrument has been subjected to thorough and extensive laboratory tests. It will generally give a clear and reliable indication of whether or not a particular gemstone is a diamond. Where any doubt exists, supporting test should be performed.



  • The probe and particularly its tip should be handled carefully. Especially when removing the protective cap from the pen top, be careful not to damage the tip. When the probe is not in use, always replace cap on the probe pen.
  • Change batteries as soon as the indicator lamp fails to come on brightly
  • Do not leave worn out batteries in the meter case as they may lead to corrosion. Batteries should be removed when the instrument is to be stored for any extended period of time.


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