The stone­-like thing in the pictures you se­nt looks like a small tooth deposit. The black line­ could be stained or cracked. Tooth deposits can form from leftove­r plaque. Stains might come from food or drink. It is good you have no pain, re­d, or swell - that is a nice sign. To fix this, make sure­ to brush and floss a lot. Also, see your dentist for a check and cle­an. They can take care of the­se things for you.