Tarnish on silver jewelry is a product of oxidation, caused by the interaction of the silver jewelry with certain elements such as oxygen or sulfur as well as with certain acids. Storage for an extended period of time causes tarnish on the silver jewelry. Contact with materials such as rubber is also a cause of tarnishing. Taking a medication that causes a change in the skin's PH can also cause silver jewelry to lose its shine.
It is normal for silver to darken, because it reacts to substances that may be in the environment, although it is also due to the PH of each person.
If you store your jewelry for a long time, it may darken. If it is 925 silver, you can clean it in the manner described below and it will be like new again.