The normal process of change is: brittle nails, straight nails, spoon-shaped nails. Subungual hematoma occurs when trauma to the nail results in a collection of blood, or hematoma, under the nail. It may result from an acute injury or from repeated minor trauma such as running in undersized shoes. Acute subungual hematomas are quite painful, and are usually treated by releasing the blood by creating a small hole in the nail. Drilling and thermal cautery  are common methods for creating the hole. Thermal cautery is not used on acrylic nails because they are flammable. Onychomatricoma, a tumor of the nail matrix. Nail pemphigus, an auto-immune disease. Erythronychia, red bands in the nail from some inflammatory conditions. Melanonychia, a black, brown or grey  discoloration of the nail, with numerous causes. Nail changes and conditions associated with them
Nail inspection can give hints to the internal condition of the body as well. Nail disease can be very subtle and should be evaluated by a dermatologist with a focus in this particular area of medicine. A nail technician may be the first to note a subtle change in nail health. Pliability
Brittleness is associated with iron deficiency, thyroid problems, and impaired kidney function. Splitting and fraying are associated with psoriasis and deficiencies of folic acid, protein, and Vitamin C.
Unusual thickness is associated with circulation problems. Shape and texture
Nail clubbing - nails that curve down around the fingertips with nailbeds that bulge is associated with oxygen deprivation and lung, heart, or liver disease.
Koilonychia - spooning, or nails that grow upwards. Associated with iron-deficiency anaemia or vitamin B12 deficiency. Pitting of the nails is associated with psoriasis. Beaus lines are horizontal ridges in the nail. Habit-tic deformity is a condition similar to Beaus Lines caused by long-term skin picking. Discoloration of entire nail bed
Yellowing of the nail bed is associated with chronic bronchitis, lymphatic problems, diabetes, and liver disorders. Brown or copper nail beds are associated with arsenic or copper poisoning, and local fungal infection. Redness is associated with heart conditions. Other color changes and markings
Melanonychia (longitudinal streaking that darkens or does not grow out), especially on the thumb or big toe, may indicate subungual melanoma. White lines across the nail (leukonychia striata, or transverse leukonychia) may be Mees lines or Muehrckes lines. Small white patches  are known as leukonychia punctata. Dark nails are associated with B12 deficiency. Stains of the nail plate (not the nail bed) are associated with  smoking, and henna use. Splinter hemorrhages (or haemorrhages) are tiny blood clots that tend to run vertically under the nails. Drug-induced nail changes are caused by drug usage which may result in various abnormalities. : 665–6

Treatment
In approximately half of suspected nail fungus cases there is actually no fungal infection, but only some nail dystrophy. Before beginning oral antifungal therapy the health care provider should confirm a fungal infection. Administration of treatment to persons without an infection is unnecessary health care and causes needless exposure to side effects.