In my finger nail there is a white spot, what could it be and why is it there and how can I prevent it?

The spots you notice are not caused by a calcium deficiency. They're called ';leukonychia'; and are very common. Most of the time the white spots simply are a sign of some past injury to the matrix (base) of your nails. By the time the white spot shows up (about six weeks after the injury) you've probably forgotten all about banging or knocking your fingers. Sometimes, the injury can stem from a manicure that put excessive pressure on the base of the nails. The spots also can be a sign of an allergic reaction to nail polish or nail hardeners and, sometimes, are a symptom of a mild infection.