Peaches are a type of fruit produced from the Prunus persica
tree, which is of the Rosaceae (Rose Family). Although the fleshy part of the fruit is edible, the stony shelled seed--or pit enclosed within, is NOT. These seeds contain traces of HCN (hydrogen cyanide) glycosides, which is toxic to all animals and humans.
Although the seeds are the main concern, there are also certain periods where parts of the peach tree can contain HCN glycosides as well. Young, rapidly growing leaves tend to contain increased levels of HCN glycosides.