The white half circle on the nail is called the lunula, which is the visible part of the matrix, the whitish crescent-shaped base of the visible nail). The lunula is easiest to see on the thumbnails and is usually a whitish color. The absence of lunulae does not always indicate a serious health problem, but it is a good idea to discuss the issue with a doctor. Here are some details about the lunula and other nail-related information:

• Lunula: The lunula is part of the matrix, which refers to the tissue just beneath the nail. It contains nerves, lymph, and blood vessels and produces the cells that become the hardened nail plate. The lunula appears white due to a reflection of light at the point where the nail matrix and nail bed meet).

• Nail growth: Nails start in the nail root, hidden under the cuticle. When cells at the root of the nail grow, the new nail cells push out the old nail cells. These old cells flatten and harden, thanks to keratin, a protein made by these cells. The newly formed nail then slides along the nail bed, the flat surface under your nails. The nail bed sits on top of tiny blood vessels that feed it and give your nails their pink color.

• Nail composition: Nails are made of keratin, which is the same substance your body uses to create hair and the top layer of your skin. The nail plate is made by the nail matrix as the cells die off, becoming progressively thinned out and broadened. They fill with keratin and even lose their nuclei. These cells are embedded within a matrix of more proteins and elements such as sulfur, calcium, iron, aluminum, copper, silver, gold, titanium, phosphorus, zinc, and sodium.

• Other nail-related conditions: The absence of lunulae may be a sign of a vitamin deficiency, vitiligo, kidney failure, or another health condition, some of which can be serious. Terry’s nails is a condition that causes the bulk of your nail to appear white, completely erasing the appearance of the lunula. It’s characterized by a pink or red band of separation near the arc of your nails. Although it can happen on one finger only, it usually affects all fingers. In older adults, this condition is usually a natural sign of aging. In some cases, it may be a sign of diabetes, liver disease, or kidney failure.