The only type of gold that doesn't tarnish is 24-carat gold. This is the purest form of gold, and it's what most people think of when they think of gold. It is also the most expensive type of gold, since it doesn't mix with any other metal. Pure gold, like 24-carat gold, doesn't tarnish because it doesn't easily combine with oxygen.
It's extremely rare to find a pure gold ring because the base metals are alloyed together with gold to create a stronger, harder ring. The base metals used are exposed to oxygen and sulfur and eventually cause gold rings to tarnish. Gold is capable of carrying even a small electrical current at temperatures ranging from -55°C to +200°C. No, a tarnished gold ring doesn't mean it's fake, it simply means you don't have a pure 24-carat gold ring.
For these reasons, gold is used in many electronic products, such as computers, high-end technical equipment, spaceships and satellites (reference). However, this is precisely what the Russian precious metals trading company, International Reserve Payment System, discovered in thousands of (supposedly) 999 St. George gold coins issued by the Central Bank of Russia (reference). Cable joining in some microelectronic applications can promote the formation of intermetallic compounds of gold and aluminum.
When gold comes into contact with other metals, such as copper or silver, it can also produce a tarnished gold color that is greenish black. Perfumes and other chemicals can corrode the metal alloys that make up gold jewelry, even if they don't react directly with pure gold. To remove tarnish found on the surface of your gold ring or gold jewelry, use a small amount of phosphate-free dishwashing liquid or warm water. Gold is the least reactive of all metals and is benign in all natural and industrial environments.
Gold never reacts with oxygen (one of the most active elements), meaning it doesn't rust or tarnish. When done correctly, gold plating prevents tarnishing by using pure gold to protect the most reactive base metal from the environment. This usually results in a thin layer or poorly bonded layers that quickly fade away, exposing the base metal underneath to a golden tarnish. To avoid introducing intermetallic compounds into circuits, the gold and aluminum components must be joined together without using heat.
Tarnish is a chemical reaction that occurs when gold comes into contact with oxygen or other chemicals in the air. Gold rings smaller than 14 carats will have less pure gold and are more likely to tarnish over time.