The fingernail, and toenail, too, is a nonliving tissue called keratin,
a protein similar to animal hooves. Keratin is made by the nail matrix
located at the base of the fingernail and covered by the cuticle.

Severe malnutrition may slow nail growth. But an illness or a disease
may hamper circulation to the nail matrix, causing the fingernail to
form abnormally.

Thyroid disease, for example, may cause a spooning of the fingernail.
And a blue appearance of the nail bed may indicate circulatory problems
seen in diabetes or heart disease.