Hard speck on my gum?

I recently noticed that there is a hard little speck thing on my bottom gum slightly under/between my two middle bottom teeth. Neither I nor anyone else can see it, yet i can feel it with my finger and tongue. It doesn't hurt at all, yet it's odd . It almost feels like a sharp little piece of bone from my jaw or the bottom of my tooth. Someone suggested that it might me tartar, but i'm not 100% sure what that is. I think i will be going to the dentist soon to have a look at it, but until then any ideas on what this might be or what i should do?