Ok so the line is really small and is brownish black. I have never noticed this before. I wonder if this is a cavity or not. I have to go to the dentist tomorrow. Is this a stain or cavity. It is in the bottom first molar. I really try to take care of my teeth well. I brush them 2-3 times a day and I have only had 2 cavities in my life time. I am a junk food junkie though and I love sweets. But I dont eat sugary junk food/candy that often. I always buy sugar free gum of i get gum. The brownish black line is in one of the groves of my tooth so I wonder if it is a stain that for caught in my fissures. So please give me answers quick because I only have a day to find out. Thanks