Discrete math

a Robin Hood and his fellows Little John and Marian snuck in to a jewelry store; one of them stole a sapphire, one stole a diamond, and one stole an emerald. They were caught and put on trial, during which they made the following statements: Robin: "John stole the sapphire." Marian: "No, John stole the diamond." John: Both of them are lying. I didn't steal either. It turns out that the one who stole the emerald lied, and the one who stole the sapphire told the truth. Who stole which gemstone