ET in the classroom: How pure is your platinum ring…

How Is Purity Measured?

Platinum is a precious metal, rarer and more valuable than gold. It is typically used in a very pure form, often as high as 95%. Platinum jewellery falls into three categories of purity, each with its own marking: Platinum, Plat/Pt, and Platinum Group Metal Mixtures.

What Do The Markings 'Platinum' And 'Plat' Signify?

Jewellery that contains at least 95% platinum (or 950 parts per thousand) can be sold as platinum. A piece of this jewellery is marked 'Platinum'. Jewellery that is 85% or 90% pure is marked as 'Plat' or 'Pt', with part per thousand being 850 or 900. For example: '900Pt.' or '850Plat.'


What Are Platinum Group Metal Mixtures?

Another type of jewellery is made from a mixture of platinum and one or more metals that are classified as the 'platinum group' metals. This group includes platinum, iridium, palladium, ruthenium, rhodium and osmium.

A piece that contains at least 50% (500 parts per thousand) of platinum and a total of 950 parts per thousand of any of the 'platinum group' metals can be marked as platinum jewellery as long as the amount of each metal is disclosed. For example, '700Plat. 200Pall. 50Irid'. But any jewellery with platinum content less than 35% should not be considered as platinum jewellery.

How is purity measured in gold?

In karats. Any gold sample will have a maximum purity of 24 karats. In practice, however, this sample will have a purity of 999.99 per thousand. There will still be 0.01 per thousand by weight of impurity left in the gold, which is due to its refining process and cannot be removed.

A gold ornament with a 16-karat tag means that this piece has 16 parts of gold and the remaining 8 parts of some base metal. So a 16-karat gold piece or ornament is 66.67% pure (16/24). The remaining 33.33% (8/24) is the impurity. Similarly, if you take a 20-karat gold sample, it means it has 20 parts of gold and the remaining 4 karats of impurity. In other words, it is 83.33% (20/24) pure and has an impurity of 16.67% (4/24). Jewellery below 9 karat does not qualify as gold even though it has a 37.5% gold content.