The Kafkania pebble is a small rounded river pebble about 5cm long with Linear B symbols and a double axe symbol inscribed on it. It was found in Kafkania, some 7 km (4.3 mi) north of Olympia, on 1 April 1994 in a 17th-century BC archaeological context. If it were genuine, it would be the earliest written testimony on the Greek mainland, and by far the earliest document in Linear B. However the inscription is possibly a modern forgery.
More information on the Wikipedia page  which gives some of the reasons for considering it a fake.