Chlorine Stress Fractures
Pure gold (24 carat) is almost completely resistant to reacting with chemicals of any nature. Aqua Regia is a specific mixture of hydrochloric acid and sulphuric acid and this can and does react with pure gold.
As pure gold is very soft, it is almost never used for jewellery as it is easily damaged by impact, force or frictional wear.
Gold alloys where other metals are mixed with pure gold are much more robust, stronger and harder, making them more difficult to damage in day to day wear.
Depending on the colour of gold desired, varying amounts of silver, copper, zinc and other metals are added to the alloy to produce a harder wearing metal, much more suitable for producing rings, chains, pendants, earrings or charms.
Although an alloy of gold is physically superior to pure gold for the purposes of creating jewellery, it becomes susceptible to chemical attacks.
Chlorine is an element in the halogen group on the periodic table.
This group also contains the elements iodine, fluorine and bromine.
These chemicals will react (in a bad way) with the other metals in a gold alloy or sterling silver.
Chlorine compounds in solution are found commonly in swimming pools, spas, cleaning products, bleaches and disinfectants.
Exposing your gold or silver jewellery to chlorine compounds is not going to end well. Damage will occur to the non-gold metals in the alloy and the jewellery may fail mechanically, most commonly at a point of stress.
The lower the carat of gold alloy and the stronger the chlorine compound, the more quickly damage will occur. An increase in temperature will also speed this process.
The concentration of chlorine in bleaches and cleaning products is commonly very high and will cause almost immediate damage to jewellery. You should always remove any jewellery that may come in contact with these products before you start to clean and thoroughly wash any traces from your hands before putting your jewellery back on.
Pools and spas use chlorine to keep the water clean and bacteria free. The concentration is lower when compared to cleaning products, but the time spent where your jewellery is in contact with chlorinated water is usually much longer. The heated water of a spa or hot tub will cause a reaction between your jewellery and the chlorine to occur more quickly.
The prongs (claws) of a ring holding a stone into the setting are (should be) under constant mechanical stress to hold the stone firmly in place. This stress and the presence of chlorine compounds make the connection between the prongs and the shank of the ring highly susceptible to chlorine stress fractures. The prong may separate from the shank and the stone has one less prong holding it in place. This is one of the most common ways that set stones are lost from jewellery. It takes a lot less force to break a prong if it gets caught on your clothing when it has been damaged by chlorine compounds.
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Sea water contains many dissolved chemicals which may cause harm to your jewellery. It is likely they are in low concentrations so the effect would be expected to minimal. The spring in a necklace or bracelet clasp is spring steel and may corrode in salt water.
Salt water pools contain chlorine. They use electrolysis to turn salt (sodium chloride) into chlorine.
The most common problem is going to be a ring slipping from a finger that has shrunk in colder water or a chain that gets pulled and breaks at the jump ring. Either way it's not going to be easy to retrieve your jewellery from the sandy bottom of an ocean or river.
Regular visits to your jeweller should reveal any damage that may have occurred and having the damage repaired before it becomes catastrophic will very likely prevent the loss of precious stones. It doesn't take long and may be done when having your jewellery cleaned.
Always remove jewellery before swimming, never expose your jewellery to cleaning chemicals, bleaches or chlorinated water.