Grimm's toys are one of a kind, carrying unique marks made by nature. Wood grain has changing structures and colours which serve as proof of its natural origins. The unique variations are proof of quality, not defects.
For example, mineral deposits can cause dark spots on a rainbow. This means that if the tree had grown further, a branch would have developed at this point.
Black stripes in wood are a natural phenomenon. The so-called pith tube or "heart of the tree" is darker in colour, due to its structure. When working on a piece of wood, it can become visible or partially visible.