The lack of nutrients in the diet, or the lack of a balanced diet, can contribute to the discoloration of the area under the eyes. It is believed that iron deficiency can cause dark circles as well. Iron deficiency is the most common type of anemia and this condition is a sign that not enough oxygen is getting to the body tissues.

The skin can also become more pale during pregnancy and menstruation (due to lack of iron), allowing the underlying veins under the eyes to become more visible.

Any medications that cause blood vessels to dilate can cause circles under the eyes to darken. Because the skin under the eyes is very delicate, any increased blood flow shows through the skin.

Periorbital hyperpigmentation is characterized by dark circles around the eyes, which are common, often familial, and frequently found in individuals with dark pigmentation or Mediterranean ancestry. Atopic dermatitis patients may also exhibit periorbital pigmentation (allergic shiners) due to lower eyelid venous stasis, and treatment is ineffective.

Melanism, meaning a mutation that results in completely dark skin, does not exist in humans. Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful ultraviolet radiation. The same ultraviolet radiation is essential for the synthesis of vitamin D in skin, so lighter color of skin - less melanin - is an adaptation for the prehistoric movement of humans away from equatorial regions, as there is less exposure to sunlight at higher latitudes. People from parts of Africa have very dark skin, but this is not melanism.

The Silkie chicken commonly exhibits this trait. In April 2015, an extremely rare black flamingo was spotted on the Mediterranean island of Cyprus.

Pili annulati (also known as "ringed hair") is a genetic trait in which the hair seems banded by alternating segments of light and dark color when seen in reflected light.

Linea nigra (Latin for "black line") is a dark vertical line that appears on the abdomen in about three quarters of all pregnancies. The brownish streak is usually about a centimeter in width. The line runs vertically along the midline of the abdomen from the pubis to the umbilicus, but can also run from the pubis to the top of the abdomen. Linea nigra is due to increased melanocyte-stimulating hormone made by the placenta, which also causes melasma and darkened nipples.

Fair-skinned women show this phenomenon less often than women with darker pigmentation. Linea nigra typically disappears within a few months after delivery.

Afro-textured hair is the natural hair texture of certain populations in Africa, the African diaspora, Australia and Asia, which has not been altered by hot combs, flat irons or chemicals (through perming, relaxation or other straightening methods). Each strand of this hair type grows in a tiny, spring-like helix shape. The overall effect is such that, compared to straight, wavy or curly hair, afro-textured hair appears denser.

Clarence (2012) suggests that afro-textured hair may have initially evolved because of an adaptive need amongst humans' early hominid ancestors for protection against the intense UV radiation of the sun in Africa. With regard to the hypothesized recent African origin of modern humans, the author argues that afro-textured hair was the original hair texture of all modern humans prior to the "Out-of-Africa" migration that populated the rest of the globe. According to Clarence (2012), afro-textured hair may have been adaptive for the earliest modern humans in Africa because the relatively sparse density of such hair, combined with its elastic helix shape, results in an airy effect. The resulting increased circulation of cool air onto the scalp may have thus served to facilitate the body-temperature-regulation system of hominids while they lived on the open savannah. Afro-hair requires more moisture than straight hair and tends to shrink when dry. Instead of sticking to the neck and scalp when damp (as do straighter textures), unless completely drenched it tends to retain its basic springiness. The trait may have been retained and/or preferred among many anatomically modern populations in equatorial areas, such as Polynesians, Micronesians, Melanesians, Australoids and the Negrito, because of its contribution to enhanced comfort levels under tropical climate conditions.

Solar purpura (also known as "Actinic purpura," and "Senile purpura") is a skin condition characterized by large, sharply outlined, 1- to 5-cm, dark purplish-red ecchymoses appearing on the dorsa of the forearms and less often the hands.

The condition is most common in elderly people of European descent. It is caused by sun-induced damage to the connective tissue of the skin.

No treatment is necessary. The lesions typically fade over a period of up to 3 weeks.

Treatment for horses with thrush includes twice-daily picking of the feet, taking special care to clean out the two collateral grooves and the central sulcus. The feet may then be scrubbed clean using a detergent and/or disinfectant and warm water, before the frog is coated with a commercial thrush-treatment product, or with iodine solution, which may be soaked into cotton balls and packed into the clefts. Several home remedies are used, such as a hoof packing of a combination of sugar and betadine, powdered aspirin, borax, or diluted bleach. It is best, however, to speak with the horse's veterinarian, to be sure these home remedies are effective and, more importantly, safe for use on horses.

Horses with thrush, or those at risk for contracting it, are best kept in a dry, clean environment. Daily cleaning of the hooves also contributes to the prevention of thrush. In general, thrush is relatively easy to treat, although it can easily return and it can take up to a year for a fully healthy frog to regrow after a severe infection.

In segmental heterochromia, sometimes referred to as sectoral heterochromia, areas of the same iris contains two completely different colors.

Segmental heterochromia is rare in humans; it is estimated that only about 1% of the population have it.

Heterochromia has also been observed in those with Duane syndrome.

Pregnancy also poses as another high risk factor for vitamin D deficiency. The status levels of vitamin D during the last stages of pregnancy directly impact the new borns first initial months of life. Babies who are exclusively breastfed with minimal exposure to sunlight or supplementation can be at greater risk of vitamin D deficiency,as human milk has minimal vitamin D present. Recommendations for infants of the age 0–12 months are set at 5 ug/day, to assist in preventing rickets in young babies. 80% of dark skinned and or veiled women in Melbourne were found to have serum levels lower than 22.5 nmol/L considering them to be within moderate ranges of vitamin D deficiency.

Livedo racemosa is a cutaneous condition characterized by a skin discoloration appearing as irregular-broken circles.

Granulosis rubra nasi is a rare familial disease of children, occurring on the nose, cheeks, and chin, characterized by diffuse redness, persistent hyperhidrosis, and small dark red papules that disappear on diascopic pressure.

This infection is caused by the fungus formerly classified as "Exophiala werneckii", but more recently classified as "Hortaea werneckii". The causative organism has also been described as "Phaeoannellomyces werneckii".

Tinea nigra (also known as "superficial phaeohyphomycosis," and "Tinea nigra palmaris et plantaris") is a superficial fungal infection that causes dark brown to black painless patches on the palms of the hands and the soles of the feet.

While it is rare before the age of 3, about 30 percent of children between 7 and 10 years of age and 45 percent of teenagers engage in nail biting. Finally, prevalence decreases in adults. Figures may vary between studies, and could be related to geographic and cultural differences. The proportion of subjects that have ever had the habit (lifetime prevalence) may be much higher than the proportion of current nail-biters (time-point prevalence). Although it does not seem to be more common in either sex, results of epidemiological studies on this issue are not fully consistent. It may be underrecognized since individuals tend to deny or be ignorant of its negative consequences, complicating its diagnosis. Having a parent with a mental disorder is also a risk factor.

There is conflicting evidence to suggest whether obesity contributes to vitamin D deficiency.Obese individuals have an increased risk of being vitamin D deficient likely caused by lack of sun exposure from reduced mobility and or low levels of physical activity. The serum levels of obese Australian were 8.3- 9.5 nmol/L lower in both genders comparable to those of healthy weight ranges. During the AusDiab study conducted throughout Australia serum levels within obese people were shown to be 57% lower than with normal weight after receiving the same amount of UV exposure.

Inconsistent to the findings of AusDiab Study, The Australian Bureau of Statistics (ABS) found there to be no correlation between weight levels and vitamin D serum level. According to ABS the Vitamin D supplementation was said to not be a contributing factor as supplement use was similar across all weight ranges.

Confluent and reticulated papillomatosis of Gougerot and Carteaud (also known as "Confluent and reticulated papillomatosis," "CRP", "CARP", "Familial cutaneous papillomatosis," and "Familial occurrence of confluent and reticulated papillomatosis") is an uncommon but distinctive acquired ichthyosiform dermatosis characterized by persistent dark, scaly, papules and plaques that tend to be localized predominantly on the central trunk.

Fleischer rings are pigmented rings in the peripheral cornea, resulting from iron deposition in basal epithelial cells, in the form of hemosiderin. They are usually yellowish to dark-brown, and may be complete or broken.

They are named for Bruno Fleischer.

Fleischer rings are indicative of keratoconus, a degenerative corneal condition that causes the cornea to thin and change to a conic shape.

Snow mold is a type of fungus and a turf disease that damages or kills grass after snow melts, typically in late winter. Its damage is usually concentrated in circles three to twelve inches in diameter, although yards may have many of these circles, sometimes to the point at which it becomes hard to differentiate between different circles. Snow mold comes in two varieties: pink or gray. While it can affect all types of grasses, Kentucky bluegrass and fescue lawns are least affected by snow mold.

Gingival fibromatosis with hypertrichosis is a cutaneous condition characterized by dark terminal hairs on the peripheral face, central back, and extremities. It is a RASopathy.

The most obvious sign of thrush is usually the odor that occurs when picking out the feet. Additionally, the infected areas of the hoof will be black in color (even on a dark-horned hoof), and will easily break or crumble when scraped with a hoof pick. When picking the hooves around thrush areas, the differences between healthy and infected areas can be seen when white/gray tissue (healthy frog) is surrounding a dark, smelly (infected) area.

Most horses do not become lame if infected with thrush. However, if left untreated, the bacteria may migrate deeper into the sensitive parts of the hoof, which will result in lameness. Then, the horse may also react when its feet are picked out, and blood may be seen.