6mm calculi can be left on its own and will be cleared off. Apart from this do detailed biochemical investigations including serum calcium, phosphorus. Consume lot of water. Avoid tomatoes and other food stuff containing oxalates. Repeat Ultrasound
after 6 months.
Most of times the cause of calculi is Idiopathic(no cause found), investigations are required to know the cause and corect it.
All the best