Learn Portuguese (Portugal): O anel é de prata.

In English, we often use materials as adjectives, like in "a silver ring". In Portuguese, materials usually act as nouns. To describe what something is made of, you use the preposition de (of) followed by the material. So, de prata literally means "of silver".

Why do we use é here instead of está?

We use é, from the verb ser, because being made of silver is a permanent characteristic of the ring. You use ser for permanent traits and materials, whereas estar is reserved for temporary states or locations.

If I wanted to say "The rings are silver," how would this sentence change?

It would become Os anéis são de prata. Nouns ending in L, like anel, become plural by dropping the L and adding -is (along with an accent mark to keep the spoken stress in the right place). Notice that de prata stays exactly the same; because it is a prepositional phrase rather than a true adjective, it does not change to match the plural subject.