If it is a cavity, then the same method is used. Decay is removed and the tooth is filled with a composite (tooth colored) material.
Many times, small notches in teeth at the gum line are not cavities. They are notches caused by tooth grinding, chewing and clenching. If the cusps of your teeth do not come together evenly, this places extra stress on teeth. All teeth flex very slightly at the gum line, but the extra stress causes the notching at the base of the crown.
These notches can be watched over time. Night guards are prescribed to reduce grinding at night. Sometimes your dentist may choose to reduce some of the tooth cusps. This can improve the way that the teeth come together.