As a DM, I was involved in running a couple of one offs with a large group of people who were new to D&D. The group consisted of a couple of DMs who each ran there own little session with a couple of players. Afterwards, the DMs got together to share notes on the interesting things.
During this one of the other DMs shared a conversation between a wizard and a barbarian at his table. The gist of it was that the barbarian wanted to keep a prize from the battle in which they had captured a gnome. They asked the wizard to transmute the gnome into stone or iron, i.e. something that wouldn't rot. The wizard said that he had no idea how to do this, but wouldn't be willing to do it even if he could, so no attempt was made to transmute the prisoner.
The DM who came up with this said he wouldn't have let it happen anyway but some of the others disagreed, saying given enough time and research they would allow it.
My question is whether or not the rules specify this anywhere? Since the principle seems sound (just think of basilisks and their) what would it entail to transmute a living creature?