Under normal conditions, gold should not change color. However, due to use and time, it can be scratched and tarnished, which can damage the aesthetic appearance of gold. Have you ever found a gold ring that had turned copper in color and wondered why? Gold jewelry discolors over time because it's not pure gold. Gold itself doesn't usually change, but it's the other components that tarnish over time.
The discoloration of gold can also be due to poor gold-plating techniques, in which the gold layer is too thin and, eventually, due to friction, wears out, revealing the base material. 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. Although most gold alloys tarnish over time, proper care and polishing can restore them to their original shine. After some time, the silver in Pachi gold jewelry can react with or with oxygen and cause the jewelry to turn 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. In general, tarnishing is mainly seen in jewelry that has less than a carat of gold and, generally, in items that have less than 14 carats of gold. However, you can protect its shine and care for your gold jewelry by purchasing a solution that removes the rusty layer that builds up over time. Perspiration, which is essentially rich in sodium chloride (common salt), can cause gold jewelry to tarnish.
These substances, after contact with gold jewelry containing silver and copper alloys, can produce a chemical reaction to produce silver chloride and copper sulfide, which can cause the creation of deep black chemical salts. But pure gold or 24-karat gold is too soft to be used in jewelry, so it is generally alloyed with other base metals. 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. Like all plated items, 18-carat gold-plated jewelry tarnishes over time and reveals the metal underneath.
Just because your 22-carat gold jewelry tarnishes doesn't mean it's not real gold; it just means that your gold jewelry isn't “pure” gold or 24-karat gold.