Petroc Trelawney was a pet rock given to Jenna by Silas and the rest of the Heap family on her 10th birthday, also the day she discovered she was a princess. He is a small ovular gray rock, nice and smooth. He was awakened by Silas using a charm he got from the Traveling Rock Shop. This brought the little stone to life.
He is awakened in the book by tickling him underneath the chin. He has eyelids that open up to little, solid black eyes. He has a little mouth and tongue within it. He also has four little stubby legs that stick out when awakened. Petroc has been known to like stickiness, eels, and cabbages. He displays this when he licks Boy 412 (later Septimus's) hand. His tongue is described as "dry and slightly rasping, like a minute cat's tongue." He will eat, but is not fond of Sally Mullin's bread. Also Petroc cannot be fed too much or he will become heavy and lazy and he also needs a daily walk.
Petroc's life began on Jenna's birthday  and he stayed with her until he was lost. As a present for becoming Queen, Wolf Boy gives Jenna a bag of pebbles that is soon revealed as Petroc and his many offspring.