Measures

• 10 Millimetres (mm) = 1 Centimetre (cm)
• 2.54 Centimetres (cm) = 1 Inch
• 1 Millimetre (mm) = 0.0393701 Inch

Weights

• 1000 Grams (g) = 1 Kilogram (kg)
• 28.3495 Grams (g) = 1 Ounce (Avoirdupois)

Other Weights
• 1 Troy Ounce (oz t) = 31.103 Grams (g)
• 1 Carat (ct) = 0.2 Grams (g) or 200 Milligrams (mg) (Used for gemstones)
• (Formula divide the mass value by 5 eg. 5gms Divided by 5 = 1ct)
• 1 Pennyweight (dwt) = 1.555 Grams (g) (Used in the US jewellery trade)

Hallmarks and Purity Stamps

Hallmarking is a legal requirement in the UK for most precious metals over a certain weight but is not required in other countries. A full UK hallmark consists of three compulsory marks:

• Sponsor's Mark: The unique mark of the company or person who submitted the item for hallmarking.
• Assay Office Mark: Identifies where the item was tested and marked (e.g., London, Birmingham, Sheffield, Edinburgh).
• Fineness Mark: Indicates the purity of the metal in parts per thousand.
• NOTE: A gold stamp such as 9ct or 9k is not a hallmark and is not a legal requirment in Australia and many other countries.

Gold Purity (Carat)

The carat (ct) is a measure of gold purity, with 24ct being the most pure gold (99.9%). The fineness mark on jewellery shows the purity in parts per thousand. The carat (ct) mark is used to denote measure of gold in most countries with the exception of the Untied States, Canada and Mexico who use karat (K)

• 9ct, 9k, 375 - 37.5% pure gold. Common in British Commonwealth countries.
• 10ct, 10k, 416 - 41.6% pure gold. The USA standard.
• 14ct, 14k, 585 - 58.5% pure gold. Common in the USA.
• 15ct, 15k, 625 - 62.5% pure gold. British Commonwealth (Discontinued in 1935).
• 18ct, 18k, 750 - 75% pure gold. A standard for fine jewellery.
• 22ct, 22k, 916 - 91.6% pure gold. Common in Asia and the Middle East.
• 24ct, 24k, 999 - 99.9% pure gold.

Silver and Platinum Fineness

• Silver 925: 92.5% pure silver, commonly known as Sterling Silver.
• Platinum 950: 95% pure platinum.

Plated vs. Solid Jewellery

The term "hallmark" legally applies only to solid precious metals. Plated items do not have a gold hallmark. Instead, they are marked according to the base metal.

• Gold-Plated: A thin layer of gold over a base metal (e.g., brass or copper).
• Gold Vermeil (ver-may): A thick layer of gold (at least 2.5 microns) plated over Sterling Silver. These items are typically marked with the silver hallmark (e.g., 925 ) to indicate the base metal but some countries may also include a gold ct mark (e.g., 925, 9ct)