There is something special about naturally formed circles. In Finnish folk magic, this is visible in the magic objects called ringbranches (Fin. umpipuu). These are usually branches or roots of trees that have grown so that they form a circle. These objects have especially been used in healing practices. A healer would pour the ingredients of the medicine through the hole in the object, often three times. In the name umpipuu the prefix umpi refers to the closed circle on the object, and it was used to heal umpitauti, where the prefix refers to the blocked state of constipation or urinary retention. When pouring medicine through the hole in the object the healer also mimicked the desirable effect of the cure.
Making medicine with a ringbranch is connected with practices of “passing through”. These practices involve pulling or passing patients through holes in trees, stones, or the earth. Some of the ringbranches in the collection of the National Museum of Finland are recorded to have been used for these kinds of practices: for example, children who were slow to learn to walk would be passed through a ringbranch.
For more information on practices of “passing through”, see Wayland D. Halm: Magical Medicine. The Folkloric Component of Medicine in the Folk Belief, Custom, and Ritual of the Peoples of Europe and America. University of California Press, 1980, pages 133-185.