Chains take a lot of abuse from daily wear. They rub against your skin, clothes, other accessories and are frequently pulled. Even gentle pressure affects their strength. Depending on current styles, chains may be sold that are actually too fine for everyday wear.
They may even be made from gold or platinum, but if they break frequently, they are experiencing a heavier use than what was intended in their manufacture.
If the chain is really thin, it is not meant for everyday wear. It cannot hold a lot of weight, i.e. a large or heavy pendant, and will most likely break over time.
To determine the quality of costume chains, examine the links. If they are not soldered shut, meaning there is a break in the links, they can easily be pulled apart. This is more of a concern for small to fine chains than larger thicker diameter link chains.
Small chains that have links less than 1mm in thickness are most secure when there is no break in them. Chains made from stainless steel or titanium are quite strong, but it is still important to make sure they cannot slip out from each other at the point where links have a break in them.
Some chains have what is called a break away jump ring on at least one side of the claps. Do not confuse this with poor craftsmanship as this is considered a safety precaution. In the event a chain becomes caught on something, the unsoldered jump ring will allow the chain to break away first before the wearer can be choked.
’20 Tips on maintaining our Jewellery’, Gillian E. Batcher, 2013