Semi-Precious Metal

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

Which metals belong to this group?

The classification varies depending on the source, but the following metals are most commonly categorised as semi-precious metals:

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

In the narrower sense of the precious metals trade, silver is usually classified among the precious metals (investment silver, coin silver), while copper is considered the classic semi-precious metal.

Electrochemical background

The classification is based on the electrochemical series: precious metals have a high positive standard potential and do not react with dilute acids. Semi-precious metals fall in a middle 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-base)       (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 — the copper price is regarded by economists as an economic indicator ("Dr. Copper").

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

In brief

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