Many old manuscripts instruct that spells be written on virgin skin, virgin parchment or skin of a virgin kid. 

What? What does that mean??

During the Middle Ages favored parchment was actually made from the skin of animals. The most common form of parchment was made from the skin of young goats (kids) or sheep. The term virgin simply meant that the piece of parchment required was to be new and unused.

Later, vellum made from the skin of calves, lambs or rabbits was introduced and was considered to be of even better quality.

The finest quality parchment, however, was uterine vellum and it was created exclusively from the skin of aborted, stillborn or newborn calves. It was a highly desired and costly product as its surface was smooth and purely white in color.

Because of how expensive the most prized parchment was, it was used cautiously and carefully.