Gem specialises in boutique sakes collected from all over Japan. There's a menu, which is always changing, but if you're here to broaden your appreciation of the myriad styles of nihonshū (and that's why everyone is here), it's best to leave the choosing to the savvy people behind the counter. Tell them what you like and don't like; you're in good hands.
Most go for ¥750 to ¥1500 by the glass; small plates of food, designed to pair well, run about the same. Cover charge ¥800. Come early or make a reservation.