Nutritional deficiencies can cause abnormal cravings. If you are deficient in iron these type of cravings will be present.  It can be a habit also. Eat fruits, nuts adequately.  Drink plenty of water. Try to control your intake of slate pencils. Its not serious but dont delay and talk about this to your family and Consult your nearby homoeopathic physician once. Homoeopathic medicine can be suggested by knowing your other symptoms. You can surely come out of this habit after homoeopathic medication.