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National Measurement Institute

Precious Metals and Stones

Download our Guide to the Sale of Precious Stones and Metals

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Any transaction involving precious stones or metals is deemed to be a trade transaction if the purpose is buying, selling or valuation of items. Even though a number of other factors may be involved in the buying, selling or valuation of a piece of jewellery (such as metal purity, colour, link type, workmanship, rarity or popularity of design), if a measuring instrument is used, its use must comply with trade measurement laws.

Precious Stones

Any measuring instrument used to weigh diamonds or other precious stones, must not have a verification scale interval of more than:

  • 10 mg
  • 0.01 metric carats (CM) where the capacity of the instrument is less than 5000 CM
  • 0.05 CM where the capacity of the instrument is at least 5000 CM.

The verification scale interval is the graduation size, specific to a scale that it will calculate weight by e.g. a gram, milligram or metric carat increment.

Precious Metals

A measuring instrument used to weigh gold, silver or other precious metals cannot be used when the measuring instrument has:

  • a verification scale interval that is greater than the verification scale interval specified for the instrument’s capacity
  • a capacity specified in the first column of the following table, unless it has a verification scale interval equal to or less than that specified in the second column.

Capacity of instrument​

Verification scale interval​

Less than 1 kg​

10 mg​

1 kg or more but less than 10 kg​

100 mg​

10 kg or more​

1 g​

Inaccuracies can occur if a measuring instrument is used below the minimum capacity marked on the data plate.

A verification scale interval is denoted as the ‘e’ value marked on the data plate of a measuring instrument. Before purchasing a measuring instrument, you should check the ‘e’ value to ensure that it complies with the regulatory requirements.

If the Buyer is not Present

If a person who is buying, selling or valuing precious stones or metals is not present during the measurement, they must be provided with a written statement of the weight of the items. 

Accuracy of Instruments

Under trade measurement legislation, owners and users of measuring instruments for trade are responsible for:

  • ensuring their instruments are properly installed and always measure accurately
  • ensuring their instruments are legal for trade use – in other words are of a type approved by the National Measurement Institute (NMI) and have been tested and verified by a servicing licensee or inspector
  • ensuring measuring instruments are suitable for the chosen purpose, used appropriately, and kept clean and undamaged
  • determining how regularly instruments are re-verified by a servicing licensee.

Where a measuring instrument is used to measure an article over the counter, the measuring instrument must be positioned so that the customer can clearly view the weighing process, the weight and the resulting price per prescribed unit.

If you sell or value precious stones and metals, you should conduct regular quality assurance checks with test weights to ensure that the instrument or scale is level, resets to zero and weighs accurately. This should be done at least once a day. It is recommended that you purchase a set of test weights appropriate for your scale from a measuring instrument supplier.

For more Information

The main laws covering trade measurement are the National Measurement Act 1960 and the National Trade Measurement Regulations 2009. For more information on:

  • the need for measuring instruments to have prescribed scale intervals, see section 18HE of the Act
  • the sale of precious stones and metals see regulations 2.24 and 2.25
  • the maximum permissible errors for classes of weighing instruments, see schedule 1, part 3, division 6 – weighing instruments of the Regulations. 

For more information contact 1300 686 664, or use our online form.