Discolouration of the Skin - Facts
Discolouration or blackening of the skin, clothes or items of jewellery can lead to doubts regarding the quality of the piece and can be unpleasant and irritating. There are two major causes of this form of discolouration: metallic abrasion and corrosion.
All metal wears, particularly when constantly rubbing against itself, such as chain links or swivels, or against other hard substances, such as talc, a very hard, fine powder. Jewellery items are generally not abraded by rubbing against the skin or clothing, since both are softer, however both may carry hard particles from cosmetics or medications, and modern textiles may contain stiffeners or fillers. The metallic particles abraded from the jewellery item appear dark grey or black because they are so finely divided, and a small amount of abraded metal creates a large amount of dust. When this dust comes into contact with absorbent or slightly greasy surfaces it can stick and form a black mark.
Pure gold is resistant to most chemicals associated with the human skin. However, other elements are used to alloy gold into the various carats, and the lower the carat, the higher the level of addition. These additions will, generally, be less corrosion resistant and can be superficially attacked by a range of relatively mild chemicals. Silver, an element added to the majority of carat gold alloys, can react with sulphur to form sulphides, which are generally brown in colour. This can occur under "dry" conditions, i.e. from the atmosphere, or under "wet" conditions, human sweat etc. Sweat is mainly water but also contains sodium chloride (salt), ammonia, urea, fatty acids and fat. It may also contain sulphur compounds and others associated with medications and foodstuffs. These essentially mild chemicals, when combined with the slightly elevated body temperature, can combine to cause surface corrosion. Carrier alcohol's in perfumes can also lead to mild corrosion occurring in jewellery items.