Pure gold (24k) is extremely soft so to make gold sturdy enough to use in jewelry, other metals are mixed with it, creating an alloy. Gold is an inert element so it doesn't tarnish, but the silver and copper in the alloy may react to the oxygen, sulphur and moisture in the air and eventually turn the gold jewelry dark.
There are rare circumstances in which alloys containing a high percentage of gold will tarnish, but in general the higher the percentage of gold in the alloy, the less likely it is that the gold jewelry will tarnish. Gold jewelry that is 14k and higher may never tarnish. If it does, the process will be much slower than the tarnishing of sterling silver.
To remove tarnish: To clean gold jewelry that does not include any soft gemstones, pearls or crystals put the gold jewelry into warm sudsy water containing a few drops of mild dishwashing liquid such as Dawn, Ivory or Simple Green, and rub gently with your fingers or a cotton swab. For more abrasive cleaning (only if needed), brush lightly with a soft baby toothbrush. Be gentle. Even with the added metals, gold is still very soft and easily scratched.
Do not use toothpaste or baking soda to clean gold jewelry. They are much too abrasive. Use only mild non-phosphate dishwashing liquid.
Rinse your gold jewelry thoroughly with warm water and dry with a cotton cloth. Avoid using paper products such as tissues because they contain particles of wood that can scratch the gold's surface.
Before storing, let the gold jewelry lie flat and air dry completely, overnight if possible. Moisture speeds the tarnishing process so it's important to make sure your gold jewelry is dry before putting it away.
Once the jewelry is dry, if it looks dull, rub it gently with a jewelry polishing cloth. Use a clean cloth, or a clean section of a previously used cloth, so that the gold is not scratched. To avoid creating a pattern, rub with the polishing cloth in varying straight lines, not in a circular motion.
Machine cleaning: If your gold jewelry still looks dark, it may need aggressive machine cleaning. Most jewelers have some type of automatic cleaning equipment, such as an ultrasonic, ionic or steam cleaner, and experience using it so they can give you advice about whether such cleaning is safe for your specific piece of gold jewelry. If it is, they can thoroughly clean your gold jewelry for you. The professional ultrasonic machines used in jewelry stores heat the cleaning solution so are more effective than the small ultrasonics sold for home use.
Professional polishing: Clean gold jewelry that still isn't looking like new may be scratched and in need of extensive polishing to be shiny again. This process is done in stages with abrasive papers, not with a polishing cloth, and is a job for your local jeweler. Such polishing will remove some gold so storing your jewelry carefully will save your gold, and save you some money, too.
When storing, keep each piece of gold jewelry separate from your other jewelry to avoid scratches. If the jewelry is a delicate gold chain, hanging it from a hook will keep the chain from getting tangled. Or stretch the chain out onto jewelers' anti-tarnish tissue, roll up the tissue, and then place the roll in your jewelry box, laying flat. That will keep the chain from getting knotted and also slow down the chemical process that results in tarnish. You may also want to put it into a plastic bag and squeeze as much air out of it as you can before closing (unless your gold jewelry includes pearls or opals; those gems need exposure to the moisture in the air to maintain their luster).
If your gold jewelry is well cared for and properly stored, you may never have to clean it. That means protecting your gold jewelry from being scratched by other pieces, keeping it in a dry atmosphere by using silica in your jewelry box or putting each piece in an airtight plastic bag, which in addition to keeping the piece dry gives the alloy metals less oxygen to react to, and keeping your gold jewelry away from sulphur producers such as rubber bands. As rubber bands age and break down they release sulphur so wrapping jewelry packages with rubber bands or having rubber bands in your jewelry box will speed up the tarnishing process. Other sulphur producers that you should not use for storing your jewelry are newspapers, non-archival cardboard boxes, leather, and silk.
As with all jewelry, do not wear gold jewelry in a swimming pool or hot tub or when using ammonia or cleaning bleach at home. Bleach, including chlorine, will attack and dissolve the metal so that the more delicate parts, such as connecting rings in a bracelet, will come undone. It's better to not wear gold jewelry in the shower either. The soap can leave a dull finish on the gold that is not easy to remove once it builds up.
These instructions about how to clean gold jewelry also apply to gold-filled jewelry. By U.S. law, gold-filled means that at least 1/20th of the weight of the piece is a layer of gold (often 14k gold) that has been bonded by extreme heat and pressure to base metal (usually brass). Through that process, the outer gold layer becomes joined with the inner core of base metal (the “filled” in gold-filled) and the result is jewelry that has the appearance of gold, but is stronger and costs much less.
It's important to clean gold-filled jewelry gently to avoid scratching the outer layer of gold. Better yet is to store the pieces so carefully they rarely need cleaning and never need any polishing other than a gentle rub with a soft cloth to remove fingerprints.