This whitish half-moon is called lunula (small moon in Latin) and it’s a very sensitive part of the nail.
The lunula is actually the visible part of the nail’s root and should not be damaged in any way because if you damage it your entire nail will be permanently deformed.
However, if you just damage the rest of the nail and it falls off or is surgically removed the lunula will remain intact and in its place. It is not actually white but only appears so when it is seen through the nail.
It’s most visible on the thumb but not everyone’s lunula is visible. In some cases, the eponychium, the thickened layer of skin surrounding fingernails and toenails, may partially or completely cover the lunula.
In alternative medicine it’s believed that the lunula can reveal some very important information about our overall health.
For example, traditional Chinese medicine believes that a lack of lunula indicates anemia and malnutrition while a pale or bluish lunula indicates possible diabetes. If a person’s lunula has reddish smudges he or she may be suffering from cardiovascular diseases.
A lack of lunula or a really small lunula usually indicates indigestion which can happen due to slow metabolism and toxin overload in the body.