Learn Italian: Cerco qualcosa di antico, e ho trovato una moneta d'argento. - I am looking for something ancient, and I found a silver coin.

When you want to describe an indefinite pronoun like qualcosa (something) or niente (nothing) with an adjective, you must put the preposition di between them. So, "something ancient" is translated as qualcosa di antico.

Why is antico masculine if the sentence later talks about a feminine coin (una moneta)?

The adjective antico describes the word qualcosa (something), not the coin. In Italian, qualcosa is always treated as a masculine noun, so any adjective attached to it with di must be in the masculine singular form, no matter what that "something" turns out to be!

Why isn't there a word for "for" after cerco?

The verb cercare already means "to look for" or "to search for." Unlike English, Italian doesn't use an extra preposition here. You just use the verb followed directly by the object, so "I am looking for something" is simply cerco qualcosa.

Why does the sentence say d'argento instead of just using an adjective for silver?