Learn French: Ces centimes en cuivre sont gardés par le collectionneur. - These copper centimes are kept by the collector.

Both en and de indicate what material something is made of. However, en is much more common in everyday French (like un pull en laine for a wool sweater), while de often sounds a bit more formal or literary. Both are perfectly correct here!

Why is there an s at the end of gardés?

This sentence is in the passive voice, using the auxiliary verb être (here as sont). In passive constructions, the past participle always agrees in gender and number with the subject receiving the action. Since Ces centimes is masculine plural, gardé takes an s.

Are centimes still used in France, or is this just about old coins?

The word centime is still used every day! Even though France uses the euro, the fractional coins are called centimes (like dix centimes for ten cents). This conveniently avoids confusion with the French word cent, which means one hundred.

Could we use de instead of par to say by the collector?

No, par is required here because keeping or guarding the coins is a specific physical action. We only use de for the agent of a passive sentence when the verb expresses a state of being, an emotion, or a general appreciation (for example, Il est aimé de tous - He is loved by all).