From Looney Pyramid Games Wiki
The right koan has the Buddha-nature, while the left one does not.

Buddha-nature is the mysterious property which separates one kind of koan from another, in Zendo. A koan which has the Buddha-nature obeys the Master's secret rule, which must describe some observable property of the koan. Conversely, a koan which breaks the rule must lack the Buddha-nature.

Traditionally, a koan which has Buddha-nature is marked with a white marking stone; a koan without Buddha-nature is marked with a black marking stone.