The official rules of golf state that a ball marker, small coin or other similar object should be used to mark the position of a golf ball. According to About.com Golf, this is a recommendation and not a requirement, so there is no penalty for using an object that does not conform to this recommendation.
A person's choice of golf marker comes down to etiquette. A small, round and flat object is the best choice, as a large or unusual ball marker can be a distraction to other players. If a person is bothered by another player's choice of ball marker, he may politely ask the other player to choose a different ball marker. If the other player declines, the person must either learn to deal with the distraction or ask the other player to move his ball marker. Official golf rules do state that no matter what is used to mark the golf ball, if it interferes with the play, stroke or stance of another player, the ball marker should be placed one or more clubhead lengths to one side until it is no longer bothersome. Even if the ball marker is simply causing "mental interference," it is within the rules to ask that it be moved.