Gold is one of the least reactive chemical elements. Gold alone or pure gold doesn't combine easily with oxygen, so it stays shiny, doesn't rust or tarnish, again, that's pure gold. In jewelry, it's rare to find a piece that contains only the element pure gold. Pure gold or 24-karat gold is too soft to be used in jewelry, so it is generally alloyed with other base metals that alter its properties.
It's these other base metals alloyed with gold that actually react with or with oxygen, sulfur, and moisture that eventually tarnish your gold jewelry. When exposed to oxygen and moisture, copper, one of the most popular metals used in 14-carat gold rings, turns green, especially when it comes into contact with the skin. As indicated by the carat number, pure gold (24-karat gold) doesn't react with oxygen, meaning it's unlikely to tarnish. There are key differences between 14-carat and 18-carat white gold, the main difference being in the purity of the gold.
Other influences that can also contribute to 14 or 18 carat jewelry tarnishing even more are constant exposure to large quantities of chemicals such as perfumes, hairspray, deodorants and cleaning products such as chlorine and detergents. If you have to bathe yourself with your gold chain, rinse it thoroughly afterwards with clean water and dry it with a soft cloth. While pure 24-karat gold doesn't react with anything, jewelry with lower carats, such as 14-karat gold rings, contains additional metals that can react with environmental factors, changing the natural color of 14-karat gold to green or black. There are even rare circumstances in which jewelry containing high carats of gold has tarnished, but in general the higher the percentage of gold in the alloy, the less likely the gold piece is to tarnish.
If you are concerned that you are allergic to 14k gold, ask your jeweler about the composition of the metal. If your gold jewelry looks dull after a long time of use, you can take it to a trusted jeweler so that an expert can clean it thoroughly. Keep all types of gold jewelry away from water in the shower, pool or ocean to maintain a brilliant shine. Pure gold is very soft for everyday use, it is common for other metals such as silver, copper, nickel or zinc to be added and converted into a more durable alloy.
Experts can only deduce that the jewelry could have been exposed to a corrosive atmosphere at some point and it doesn't really mean that the gold in these items is fake. Blackening of 21- and 22-carat gold objects has been reported, but these are very unusual and truly unexpected cases. Gold tarnishing is the slight corrosion of the surface of gold and is evident as a dark discoloration of the gold item, also called tarnished film.