Is this any disorder or deficiency

hi friends,

I have a friend who never consider this as a problem but i think he is suffering from anemia or some liver problem. He likes to eat Chalk pieces which we use to write on black board or slate. I saw him eating a whole chalk at one time. When i asked that how is he able to eat he told i like to eat them. I told him it must be some deficiency but he saw now i am fine and i eat food properly that how can you say that i am having problem. Is he suffffering from any disease and why is this chalk not harming his body.