My daughter likes to chew on pebbles.
Putting non-edible items in your mouth is sometimes known as 'pica'. Your daughter may be getting some positive sensory input through chewing the pebbles. She may need to be given something which has a similar texture to them for her to chew on instead, such as boiled sweets or a teething ring which is hard and has a smooth surface. You may find she needs to chew on something at certain periods of the day, perhaps when she is particularly anxious or stressed.
Some parents will seek an occupational therapist's (OT) assessment if their child likes to chew on different inappropriate objects. You can ask your social worker or GP for a referral to an OT (please see 'Behaviour: useful contacts/resources').
What is important is to work out what your daughter is getting out of chewing pebbles. Is it that she gets a lot of attention when she does this? If so, try at act as neutrally as you can when she does it. A firm 'no' with little eye contact, reinforcing it with a symbol, may reduce the excitement.