Pica is characterized by an appetite

Appetite

The appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...

 for substances largely non-nutritive (such as clay or chalk). For these actions to be considered pica, they must persist for more than one month at an age where eating such objects is considered developmentally inappropriate. There are different variations of pica, as it can be from a cultural tradition, acquired taste or a neurological mechanism such as an iron deficiency, or chemical imbalance.