Why does certain jewellery "make you black"?

When one buys jewellery of a low gold content, you have to be aware of certain drawbacks. It may happen that the wearer notes a certain decolourization of the jewel on the skin or clothing or spots on the jewel itself. This may happen in particular in the case of 9ct gold. The explanation of this phenomena is quite easy. As already explained, gold alloys contain oxidable metals. For example: silver is easily attacked by substances containing sulphur and resulting in a brown or black colouring. Copper, like silver, reacts with sulphur and also with ammonia. These oxidising substances are contained in the sweat, in certain skin creams, soaps, perfumes and fabrics. In low-content gold alloys we have now a large proportion of copper reacting not only in the above mentioned cases, but also with ammonia and sulphur contained in the air. A similar reaction may be the result of :
 

a) the secretion of the wearer’s skin
 

b) cosmetic or textile products used
 

c) external influences
 

Oxidation phenomena have also been observed on 8 to 14ct gold alloy watch cases caused by oxidising substances secreted by the strap leather, the glue used or certain synthetic packing materials. This phenomenon, however, rarely happens with 18ct alloys. It may nevertheless happen in the case of  massive jewels composed of many rings or hinges. The friction between the moving parts will produce the abrasion of metal particles which are easily oxidised and produce black spots or streaks on skin and clothing. The softer the alloy, the more probable will be the possibility of observing this phenomenon. It is easily overcome by applying a hard gold plating.