Marking Ball on the green

Question
Several years ago I seemed to remember a player being penalized for using his room key for a marker on the green.  Yet everything I read now says there is no restriction on what you use to mark your ball.  Has this rule changed?

Answer
Robert,

The rule book states the the ball should be marked with a ball marker, small coin or similar object. I remember when players would use their hotel key or a tee to mark the ball. However that is no longer acceptable.

Thank you,

Michael