That mysterious thing that's not a rock and not bark may be a ‘gall’. A gall is a growth on a plant made of plant tissue but caused by another organism.
Insects, bacteria, fungi or viruses can all create galls. Insect galls are the most common; an estimated 133,000 insect species in the world can create galls. As you’ll see in the examples below, galls vary in size and shape; nature is very creative!
All galls are formed to assist the insect, bacteria, fungi or virus creating the gall in some way. In ecological terms, it’s a parasitic relationship. The “gall causer” manipulates the plant tissue for itself, but the plant receives no benefits in exchange.
However, most galls don’t harm the plant and have no effect on the health of the host tree, and they're perfectly natural!