Erlaubte Ball-Marker

Here's an actual USGA ruling on the subject:

20-1/16 Method Used to Mark Position of Ball

Q. The Note to Rule 20-1 provides that “the position of a ball to be lifted should be marked by placing a ball-marker, a small coin or other similar object immediately behind the ball.” Is a player penalized if he uses an object which is not similar to a ball-marker or small coin to mark the position of his ball?

A. No. The provision in the Note to Rule 20-1 is a recommendation of best practice, but there is no penalty for failing to act in accordance with the Note.

Examples of methods of marking the position of a ball which are not recommended, but are permissible, are as follows:
• placing the toe of a club at the side of, or behind, the ball;
• using a tee;
• using a loose impediment;
• scratching a line, provided the putting green is not tested (Rule 16-1d) and a line for putting is not indicated (Rule 8-2b). As this practice may cause damage to the putting green, it is discouraged.
However, under Rule 20-1 it is necessary to physically mark the position of the ball. Reference to an existing mark on the ground does not constitute marking the position of a ball. For example, it is not permissible to mark the position with reference to a blemish on the putting green.