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There can be three possibilities, however without clinical evaluation and inspection it will not be possible for me or anyone to give pin-point diagnosis.
1) Presence of calculus- Calculus/tartar which are the calcified deposits on the teeth that are formed by the continuous presence of dental plaque. It is mineralized form of plaque which develops in absence of adequate oral hygiene and can take many months to form and appear evident.
It is firm and adherent tot the teeth, is black or dark brown in appearance and is not easily removed by home care regimes of brushing and flossing alone.
Your dentist can easily remove them with the help of special instruments/ultrasonic scalers.
2) Presence of stains:
Any extrinsic stains (present on the tooth surface) will be hard to remove just with simple brushing and flossing and will require a professional visit for there removal by ultra-sonic scalers.
3) Presence of cavity/tooth decay –
A cavity /tooth decay is brown or black in color and is difficult to remove while brushing.
Your dentist can visualize and clinically look at it with the help of diagnostic instruments and if there is a definite catch in the tooth the tooth,it indicates that the tooth has decayed.
It can be filled with white tooth color filling materials which will appear as good as your natural tooth.
So I will recommend you to see your dentist and get your teeth evaluated for the proper treatment.
I hope this helps!
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