Semi-Precious Metal

The term semi-precious metal is not a strictly standardised category of chemistry but a common umbrella term for metals that sit between the fully corrosion-resistant precious metals and the base metals (such as iron or aluminium). IUPAC does not formally recognise the category; in practice, however – for example in the jewellery and coin trade as well as in metallurgy – it is nonetheless widely used.

Which Metals Belong to It?

The classification varies from source to source, but the following metals are most often categorised as semi-precious metals:

• Copper (Cu) – the best-known semi-precious metal; in air it forms a protective patina (verdigris) but dissolves in dilute acids.
• Silver (Ag) – depending on the view, also classified as a precious metal; it tarnishes in air and in sulphur compounds (tarnishing).
• Lead (Pb), tin (Sn), bismuth (Bi) and mercury (Hg) – further frequently cited representatives.

In the narrower sense of the precious metals trade, silver is usually assigned to the precious metals (investment silver, coin silver), while copper is regarded as a classic semi-precious metal.

Electrochemical Background

The classification is oriented towards the electrochemical series: precious metals have a high positive standard potential and do not react with dilute acids. Semi-precious metals lie in an intermediate range – they are more resistant than iron but less inert than gold or platinum.

Reactivity (simplified):
Gold / Platinum  →  Silver / Copper  →  Lead / Tin  →  Iron / Aluminium
   (noble)           (semi-noble)        (semi-noble)      (base)

Significance for the Precious Metals Market

Copper plays a dual role in the precious metals market: as an alloying metal it gives gold and silver hardness and colour (e.g. red gold, 835 silver). As an independent asset class it is traded on the London Metal Exchange (LME) and COMEX – economists regard the copper price as an economic indicator ("Dr. Copper").

Silver moves closely in price to the spot price of the gold market and is set in relation to gold via the gold-silver ratio. Anyone wishing to calculate the melt value of alloys of copper or silver will find a dedicated calculator for it.

In Brief

Semi-precious metals sit chemically between precious metals and base metals: they corrode under certain conditions but are considerably more resistant than iron. For investors and collectors, copper and silver are the most relevant – as alloying partners, industrial metals and independent investment options.