quiz of the day: guess card

Once upon a time at Stanford, a professor gave a test to 2 students: Matt and Ruth. The professor picked a card from a deck of 18 cards. He told Matt the suit of the card, and told Ruth the number of the card. Then the professor showed both Matt and Ruth all the 18 cards:

Heart: A, 9, Q
Clubs: J, 2, 3, 4, 7, 8, 9
Spade: 5, 6, 9, 10, Q, K
Diamond: A, 10

After showing the cards, the professor asked both Ruth and Matt what card he picked. Ruth said, "I have no idea what the card is". Matt then said, "I knew that Ruth doesn't know which card the professor picked.". Enlightened , Ruth said, "Thanks for tips Matt, I know what the card is!". Matt thought about it for a quick second and told the professor, "I know it as well!".

Which card did the professor pick?