The beautiful markings in the spalted silver birch wood I use are caused by a natural, non-toxic fungus. As the wood dries, the fungal growth stops, way before it's even considered for spoon carving.
Once carved and dried, the spalted wood soaks up more oil than non-spalted birch, meaning that as the oil oxidises and dries, it can help strengthen the weaker wood structure that has been left after the fungus has passed through.
Sought after by woodworkers, the unique colouration and patterns of spalted wood will develop their wonderful patinas with age.