Can anyone explain this “engraved” circle in my tooth? (Photos)

Hard to tell exactly why your missing enamel on your lower incisor. If it hasn't always been there, it may be the result of trauma. By the look of your other teeth, I see transverse lines that look like scratches. Did it you have braces at some point? Many times when brackets are removed, some enamel can be damaged. A simple fix for this is some cosmetic bonding which can be done in minutes!

Hi:From the photo it appears there is a missing enamel in the tooth.  If this is recent, it may be as a result of trauma.  Carbonated drinks and lemon sometimes causes enamel damage as well.Best,Dr. Maddahi

It appears for some reason the enamel in that area is missing. This most commonly can happen from trauma or normal wear and tear from eating or brushing. The location and appearance is a bit unique in your circumstance so its hard to say what caused it without a full investigation or history. Either way the fix is relatively simple and and dentist should be able to get it repaired. 

This "circle" is a localized area of slightly underdeveloped enamel. In medical terms it is referred to as enamel hypoplasia. Such condition appears as a result of developmental disruption when a tooth enamel is being formed and undergoes mineralization. The condition may vary in its severity so obviously you have a very mild and localized incidence and therefore in such case it normally does not require any treatment. Thank you for a good question and great photos!! Dr. Daniyar