Guessing stone

From Looney Pyramid Games Wiki
Some Zendo stones

In Zendo the Master gives these stones to students who guess correctly during a Mondo. At the end of your turn, you can redeem guessing stones with the Master in return for opportunities to guess at the Master's secret rule.

In the official Zendo set from Looney Labs, these stones are green, though any color other than the two colors used for marking stones is fine.