Does that mean it's not silver? Not necessarily, but first, let's be clear '925' alone (without a full set of markings) doesn't mean anything. It's not a legally recognised mark. You can buy a 925 stamp online for just a few pounds. The only way to be sure of the purity of a metal is to get it assayed by one of the recognised independent assay offices. Hallmarking is a huge topic that I will leave for its post but owning a piece of jewellery stamped 925 neither proves nor disproves its silver. That said, most rings fall under the 7.78g threshold for hallmarking silver so don't be put off if you see nothing at all.
Sterling silver contains 92.5% silver and 7.5% other metals (usually copper)
The copper gives strength and makes silver more wearable, but over time the copper content reacts to the air around it and turns black. This is easily removed with a polishing cloth but is better prevented by storing unworn jewellery in a zip-lock bag. My jewellery comes with a little anti-tarnish tab which is perfect for storing with your jewellery, but if you don't have one then a small square of newspaper will do the job. For most of us, just wearing your silver Is enough to stop tarnishing; however, there are those whose unique body chemistry reacts to the copper content, turning the skin green. You will know if you're one of these people as anything sterling silver will have the same reaction. Some people's sweat is more acidic than others and there's not much you can do about it, although painting the inside of a ring with clear nail polish, creating a barrier, can help.It's advisable to always remove jewellery when using creams, lotions, cleaning products or swimming. Chlorine especially will fast-track a black coating on your silver as I recently discovered on holiday.
If you've never reacted before and can rule out wearing your jewellery when you shouldn't be then its possible you've been mis-sold and the piece is actually plated. It's possible to pay the assay office to test this for you but unless the item cost you a substantial amount of money then I would suggest its probably not worth the expense (for silver at least).
If you find that you react to sterling silver then another option would be to buy jewellery made from Argentium. Argentium is a different alloy of sterling silver but the copper content is replaced with metalloid germanium which doesn't tarnish......more on Argentium soon.