The white discolouration on the nails like white lines is called as leukonychia. It is not because of calcium deficiency. It is a myth, there are many various reasons for it, the common reason is trauma, an injury. A old injury which can lead to this white discolouration in the nail bed or nail plate. The other is underlying systemic disorder like cirrhosis of the liver. Also can cause, there are also many other diseases which can cause, it is called as cirrhosis of the liver. Sometimes it could be even hereditary, called as autosomal dominant condition, sometimes there are heavy metal poisoning like lead, can cause this, but calcium deficiency cannot cause this whiteness of the nails.
source
