I don't quite remember having this piece of bone on my canine tooth, and yesterday I saw it. imgur: the simple image sharer (http://imgur.com/MHg65JE) as you can see, the lower canine tooth has a little bone grown over there. Any idea what that is ?

It isn't bone, and is not possible. Your teeth are anchored in bone. What you are probably seeing is a bit of receding gum line with the portion of the tooth, that is under the gumline, exposed. You obviously have braces on your teeth, so next time you are at your orthodontist, point this out to him if it alarms you

I am talking about the upper part of the lower canine tooth

Bone won't grow on teeth. If you are doubtful, ask your dentist so he can clearly see what you are looking at

It actually looks like tooth enamel or ortho cement. Check with your orthodontist.