Marking stone

Some Zendo stones.

The white and black stones are used for indicating the Buddha nature (or lack thereof) of a koan, while green ones are earned via mondos and spent to guess at the secret rule.

In Zendo, stones are used to mark koans, indicating whether or not those koans have the Buddha nature. Usually, black and white stones (like Go stones) are used, with white stones used for "has the Buddha nature" and black stones used for "does not have the Buddha nature."

The stone is never part of the koan, which includes only its pyramids.