Neko na Kankei [PSOne Books] Japan

Neko na Kankei is a visual novel game in which the player takes the role of a normal highschool student's life who always seems to carry only 30 Yen (US$0.25) with you. Then, one day, he found a stone which looks exactly like a cat's eye. With that, his normal highschool student's life changed a little. The stone has the magical ability to change him into a cat and back whenever he wants.