Semi-Precious Metal

The term semi-precious metal is not a strictly standardised category of chemistry but a common collective term for metals to be placed 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 - for instance in the jewellery and coin trade and in metallurgy - it is nonetheless widely used.

Which metals belong to it?

The classification varies by source, but the following metals are most often classed 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 named 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 to the electrochemical series: precious metals have a high positive standard potential and do not react with dilute acids. Semi-precious metals lie 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
   (precious)        (semi-precious)     (semi-prec.)      (base)

Significance for the precious metals market

Copper plays a double 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 a standalone asset class it is traded on the London Metal Exchange (LME) and COMEX - the copper price is regarded by economists as a business-cycle indicator ("Dr. Copper").

Silver moves in price closely 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 stand chemically between precious metals and base metals: they corrode under certain conditions but are markedly more resistant than iron. For investors and collectors, copper and silver are the most relevant - as alloying partners, industrial metal and standalone investment option.