This mysterious rock is the property of a magical being known as "The Lepra-Duck." It has the ability to grant the wishes of its user when those wishes are stated out loud, and it seems to have little to no limits, even allowing those using it to travel back in time and adopt the persona of one who might have existed in historical times, as can be seen when it sent Donald, Huey, Dewey and Louie Duck to the early 100's and made Donald into a contemporary Roman centurion.
The Lepra-Duck claims that there is no such thing as luck, and that his stone is the source of Gladstone Gander's infamous luck, as the stone was, from an unknown point until 1966, in Gladstone's possession. However, the Lepra-Duck hates boasting, so when he finally had enough of Gladstone's signature bragging, he took the stone back, causing Gladstone to throw a tantrum and sadly declare himself luckless. The Lepra-Duck then briefly gave the stone to Gladstone's cousin, Donald Duck, only to take it back again when Donald began to boast. It is unknown if he ever returned it to Gladstone, but many events presumably occurring after 1966 have still shown Gladstone to be supernaturally lucky.
Behind the scenes
Lepra-Duck's Wishing Stone first appeared in May of 1966 in The Battle at Hadrian's Wall, where it is revealed to be the source of Gladstone Gander's luck and is seen allowing Donald, Huey, Dewey and Louie Duck to time travel.