The image above shows a severe buildup of hardened plaque. The white material you see on all the front teeth is as hard as a rock, and must be scraped off the teeth by a dentist or a hygienist. This stuff started life as the soft plaque that can be easily scraped off the teeth with a fingernail, but it has remained on the teeth so long that it calcified. the structure between the two central teeth is called a calculus bridge. The reason that it is cracked in the middle is that all teeth move slightly, and this movement did not allow complete fusion of the calculus projections from each tooth as they grew to meet in the middle.
Notice that the gingiva (gums) around the calculus is red, while the gingiva surrounding the teeth without the calculus is pink. The redness is an indication of inflammation, and chronic inflammation like this eventually causes periodontal disease.