I recently bought some organic cinnamon (not ground, broken pieces for a hand mill) and saw what looks like pieces of wood in the mix (they don't have the same color as the rest of the pieces). Is this normal and/or safe, or should I return the product?
As Megasaur said, the spice cinnamon (or often, cassia, which is quite similar) is the inner bark of a tree. As a natural product, there is going to be some variation in the color in any case.
I would look quite closely at the texture and grain of the differently colored pieces compared to the more normal pieces. If they show the same pattern or structure, you probably just have normal color variation. If they are substantially different, then perhaps the product should be discarded or returned--or just those pieces picked out.
The purpose of this inspection, such as it is (and I am not botanical expert) is try to determine if the different pieces are still the inner bark, and not something else. I would suggest only buying spices from a reputable source in the future.