Thermal Probe

Thermal Conductivity: Heat conduction by a material has been used primarily in the identification of diamond from its simulants.

  • Breath Test is based on a diamond's high heat conduction (the rapid dispersion of the film of moisture on the diamond's surface acting as an indicator). This test has been used with varying degrees of success for many years.
  • At room temperature, the thermal conductivity of:
    • Single crystal diamond varies from 1000 W/m/°C for Type I material to 2600 W/m/°C for Type IIa material. (Watts/metre/degree Celsius)
    • Synthetic cubic zirconia, one of the early man-made diamond simulants, has a very low thermal conductivity of 10 W/m/°C.
    • All other possible simulants, corundum has the highest value at 40 W/m/°C.
  • The most recent of simulants is synthetic moissanite which has a thermal conductivity very similar to diamond and as a result gives the same reaction to the thermal testers, as diamond.
  • The instrument used to measure thermal conductivity is the Thermal Probe. It consists of a pen type test probe and a battery operated control unit.


  • The copper tip is in thermal contact with two miniature thermistors (i.e. ceramic elements) whose resistance varies inversely with their temperature.
  • If the tip is pressed against the facet of a diamond, each short pulse of heat is conducted away and the temperature of the tip falls.
  • If the probe tip is held in contact with a poor heat conductor, such as a diamond simulant, the tip temperature rises to an intermediate level which is significantly higher than that for diamond.
  • While testing a stone, the probe tip must be held in contact with, and at right angles to, the surface of the gem, for at least three seconds until a constant reading is obtained.
  • Two test surfaces are provided on the instruments control panel for checking calibration.
  • The copper tip in the test probe is spring-loaded and retracts into the body of the probe when pressed against the surface of a gemstone.
  • The Moissanite Tester was developed and marketed by C3 Inc. The C3 tester is used to identify diamond from synthetic moissanite after both give a positive reaction to the thermal probe. This tester determines the relative transparency in the near ultra-violet, where diamond transmits while synthetic moissanite absorbs and indicates the same on the instrument. A word of caution, heavily included diamonds may not transmit and may give an incorrect response.

  1. Electrical Properties: These properties are observed in gemstones in various ways.
    • Tribo-electricity or frictional electricity is the property possessed by certain gem materials whose surface develops an electrical charge when rubbed. E.g. amber and plastic
    • Pyroelectriclty is the property possessed by certain gem materials which develop an electrical charge when heated e.g. tourmaline and quartz.
    • Piezoelectricity is the property possessed by certain gem materials which develop an electrical charge when pressure is applied mechanically in certain directions e.g. tourmaline and quartz.
    • Electrical Conductivity: Most gemstones are insulators with a few exceptions. One characteristic example is that of natural Type II b blue diamonds which contains boron as an impurity and is a semi-conductor, while irradiated blue diamonds are not. This can be measured with a conductivity meter. A word of caution, natural coloured Type I a blue diamonds have been found in Australia which are not semi-conductors.
  2. Magnetic Properties: The reaction of a gemstone to a magnetic field may be either attraction (paramagnetism) or repulsion (diamagnetism).
    • Paramagnetism: Paramagnetism is possessed by iron, steel, nickel, cobalt and several gemstones. Magnetic susceptibility or the strength of magnetic pull varies from mineral to mineral and this can be measured by using a balance and a magnet. This property can be used to differentiate between similar looking gemstones e.g. spessartine from hessonite or zircon.
      • Gemstones which contain iron or manganese have magnetic properties.
      • The percentage of iron in a gemstone is not the sole cause for magnetic susceptibility. E.g. magnetite is strongly attracted to a magnet while pyrite (46% iron content) and hematite (70% iron content) are unaffected by a hand magnet.
      • Hematine, an imitation of hematite is attracted to a hand magnet. Though most hematite does not have magnetic susceptibility, material from some sources is attracted to a hand magnet.
    • Diamagnetism: A material is repelled by a magnet e.g. bismuth.