Gold has seduced the world with its beauty, but also because not even acid can alter its natural properties. An estimated 130,000 tons have been extracted from the earth since prehistoric times, of which 100,000 tons in the twentieth century alone. Gold is a malleable substance (with a hardness of just 2.5) and therefore easy to work with. It can be used in an alloy with other metals, often silver and copper. These alloys increase its resistance and change its colour.
An 18k (karat) or 750 gold alloy is generally yellow, pink, red or white. A colour standard exists for the first three shades. Each has the same base of 750 parts per thousand of gold; the yellow, pink or red colour is obtained by varying the proportion of silver or copper that make up the remaining 250 parts. White gold is an alloy with palladium (previously nickel) and is frequently rhodium-plated to enhance its whiteness. Thanks to the phasing-out of nickel in white gold in the early 1990s, and the advent of white gold solder in the same colour as the alloy, large items can be made in white gold without rhodium plating which, being only a thin layer, has the disadvantage of rubbing off locally over time.
Other colours of gold exist but are rarely used:
- Blue gold: an alloy of gold and iron. Heat treatment oxidizes the iron molecules at the surface of the metal, producing the blue colour.
- Green gold: an alloy of gold, silver and copper.
- Black gold: obtained by means of chemical vapour deposition (similar to PVD) of atoms of gold, carbon and other metals. The black coating is just a few microns thick. Other surface treatments use electrodeposition of rhodium, chromium and very dark impurities.
- Brown gold: obtained by chemical treatment.