I picked this neat ring up for a couple bucks and it was advertised as "silvertone", however, it does have a mark inside as well as some possible corrosion. Doesn't really feel like silver. What say the masses?
You will find many jewellery sites insisting that the 'S925' mark has the same status as a '925' mark, but that's a misleading simplification. It depends on where the ring was made and with what degree of regulation on the way it was sold.
In this case, your ring is not silver. It appears to be a variant of what are sometimes called 'loyal/lone wolf' rings. They're cheaply made (usually in China) and mostly sold via internet outlets. Despite the 'S925' mark (sometimes referred to as 'Chinese Silver') they're made from base metal alloys and plated.
This one from 'wish.com' available for 2GBP (approx $2.75):