Most modern commercially made jewelry should have a hallmark. There are other marks that represent silver besides 925 and sterling though. There are also different contents of silver besides 925. An easy process of elimination is to stick a magnet to the jewelry proclaimed to be sterling. If it sticks, then it's most definitely NOT sterling. The mag not sticking does not guarantee that it is silver though. There are methods for testing sterling, like adding a drop of nitric acid and checking for identifiable colors like red after the acid is applied. For me though, I've handled so much silver that I rely on my eyes to tell me whether it's silver or not. Sounds like you might have been scammed. Was it a seller from China? Can you get a refund?
silver polish does not tarnish the silver, it causes it to shine more brightly. Be careful about using the wrong polish though, because it can damage the silver. Perhaps you're not seeing the hallmarks? On lobster claw clasps, there is usually a tiny 925 mark on the locking mechanism.
it should be hallmarked, maybe the hallmark is very small, look at the clasp and the surrounding links.
Any jeweller you ask will test it for free in a few seconds. Anyway, as silver is no unexpensive compared to gold, I do not see any reason for a retailer to loose its reputation selling a fake silver chain.
a seller doesn't have to stamp their jewelry ,but there are laws to protect the consumer and so stamping is the best way to control misrepresentation either on purpose or by accident of purity content.
i would contact the selling company.Very little if any of the sterling jewelry that is sold in the department stores mentioned is made in the united states and in the many years Ive been in the trade have i seen mass production jewelry never stamped for purity.Its mandatory that imported jewelry items have metal purity stamped on it and place of manufacture either stamped on it or a tag placed on it.I think either the stamps are not visible or they are from the costume jewelry department.Most of the chain bought for the American market is made in Italy,Canada,Turkey and some from Israel and china.But most is Italian made and stamped for purity and location.
Try using ZAM on a cotton shop rage from Wal-Mart automotive, this will put a mirror shine on ANY metal cleaning off the oxidation. It even works to buff scratches out of softer stones like turquoise too. A miracle polish, lol.