Learn Italian: Il testimone deve comprare gli anelli d'oro. - The best man must buy the gold rings.

Testimone literally means "witness." At an Italian wedding, the bride and groom each have a testimone who signs the marriage register to officially witness the union. Depending on whose witness it is, this translates to "best man" or "maid of honor" in English.

Is testimone always a masculine noun?

No, it can be masculine or feminine depending on the person, but the word itself doesn't change its ending. It is an invariant noun. We know it means the best man here because of the masculine definite article il. If it were the maid of honor, it would be la testimone.

Why is it d'oro instead of a specific adjective for "golden"?

In Italian, to express what material an object is made of, you almost always use the preposition di (of) followed by the noun for the material. Since oro (gold) starts with a vowel, di drops its i and takes an apostrophe, becoming d'oro.

How would this sentence change to use the causative "fare" if the best man has his brother buy the rings?