The power of the objet trouve is with us all the time. This is from today's Guardian a British national newspaper. This image is deep within the western worlds Psyche.
So I really do not see any problem with a flint stone looking like a birds head or skull, or fitting a cultural image in the least. Especially when dealing with an animistic society where the forms are of known and recorded significance. Just look at the first Biro-Biro stones at the start of the thread.