Periorbital dark circles (also known as dark circles, infraorbital venous stasis or periorbital hyperpigmentation) are dark blemishes around the eyes. There are many causes of this symptom, including heredity and bruising.

Any condition that causes the eyes to itch can contribute to darker circles due to rubbing or scratching the skin around them. Hay fever sufferers in particular will notice under-eye "smudges" during the height of the allergy season.

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.

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.

Melanism is a development of the dark-colored pigment melanin in the skin or its appendages and is the opposite of albinism. The word "melanism" is derived from the ("black pigment").

Pseudo-melanism, also called abundism, is another variant of pigmentation, characterized by dark spots or enlarged stripes, which cover a large part of the body of the animal, making it appear melanistic.

A deficiency in or total absence of melanin pigments is called amelanism.

The morbid deposition of black matter, often of a malignant character causing pigmented tumors, is called melanosis. For a description of melanin-related disorders, see melanin and ocular melanosis.

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.

In many post-Columbian, Western societies, adjectives such as "wooly", "kinky", "nappy", or "spiralled" have frequently been used to describe natural afro-textured hair. More recently, however, it has become common in some circles to apply numerical grading systems to human hair types.

One popular version of these systems classifies afro-textured hair as 'type 4' (straight hair is type 1, wavy type 2, and curly is type 3, with the letters A, B, and C used to indicate the degree of coil variation within each type), with the subcategory of type 4C being most exemplary of this hair type (Walker, 1997). However, afro-textured hair is often difficult to categorize because of the many different variations among individuals. Those variations include pattern (mainly tight coils), pattern size (watch spring to chalk), density (sparse to dense), strand diameter (fine, medium, coarse), and feel (cottony, wooly, spongy).

The chart below is the most commonly used chart to help determine hair types:

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.

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.

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.

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".

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.

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.

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.

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.

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.

Eye injury and head trauma may also coincide with a black eye. Some common signs of a more serious injury may include:

- Double vision

- Loss of sight and/or fuzzy vision could occur

- Loss of consciousness

- Inability to move the eye or large swelling around the eye

- Blood or clear fluid from the nose or the ears

- Blood on the surface of the eye itself or cuts on the eye itself

- Persistent headache or migraine

Heterochromia is a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is a result of the relative excess or lack of melanin (a pigment). It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury.

Heterochromia of the eye (heterochromia iridis or heterochromia iridum) is of three kinds. In "complete heterochromia", one iris is a different color from the other. In "segmental heterochromia" or "sectoral heterochromia", part of one iris is a different color from its remainder and finally in "central heterochromia" there are spikes of different colors radiating from the pupil.

Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia. This is due to a mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity.

Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, usually, an excess of melanin indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia. The term is from ancient Greek: ἕτερος, "héteros" meaning different and χρώμα, "chróma" meaning color.

Thrush is a very common bacterial infection that occurs on the hoof of a horse, specifically in the region of the frog. The bacterium involved is" Fusobacterium necrophorum", and occurs naturally in the animal's environment — especially in wet, muddy, or unsanitary conditions, such as an unclean stall — and grows best with low oxygen. Horses with deep clefts, or narrow or contracted heels are more at risk of developing thrush.

Heterochromia is classified primarily by onset: as either genetic or acquired.

Although a distinction is frequently made between heterochromia that affects an eye completely or only partially (segmental heterochromia), it is often classified as either genetic (due to mosaicism or congenital) or acquired, with mention as to whether the affected iris or portion of the iris is darker or lighter. Most cases of heterochromia are hereditary, caused by certain diseases and syndromes. Sometimes one eye may change color following disease or injury.

Punctate inner choroiditis (PIC) is an inflammatory choroiditis which occurs mainly in young women. Symptoms include blurred vision and scotomata. Yellow lesions are mainly present in the posterior pole and are between 100 to 300 micrometres in size. PIC is one of the so called White Dot Syndromes. PIC has only been recognised as a distinct condition as recently as 1984 when Watzke identified 10 patients who appeared to make up a distinct group within the White Dot Syndromes.

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.

• Typically affects short sighted (myopic) women. (90% of cases are female).

• The average age of patients with PIC is 27 years with a range of 16–40 years.

• Patients are otherwise healthy and there is usually no illness, which triggers the condition or precedes it.

• The inflammation is confined to the back of the eye (posterior). There is no inflammation in the front of the eye (anterior chamber) or vitreous (the clear jelly inside the eye). This is an important distinguishing feature of PIC.

• It usually affects both eyes.

• The appearance of gray-white or yellow punctate (punched out) areas (lesions) at the level of the inner choroid. These lesions are typically located centrally at the back of the eye (posterior pole).

Symptoms typically include:

1. Blurring of vision

2. Partial ‘blind spots’ or scotoma. These areas of diminished or lost areas of the visual field are typically near the centre of vision but occasionally can be peripheral. These may be temporary or permanent.

3. Seeing flashing lights. This is known as photopsia.

The PIC lesions, which form scars deep in the choroid layer of the eye, may result in new blood vessels forming. These can be seen as the body’s attempts at repair, but these new blood vessels (neovascularisation) are weak, can spread to form a membrane and can threaten the vision. It is

suspected that at least 40% of patients with PIC develop CNV (choroidal neovascularization). This is a complication, which can occur in other white dot syndromes and other eye conditions such as macular degeneration but occurs rarely in other forms of uveitis.

CNV is a sight threatening complication and so must be picked up early and always treated. It may occur whether the uveitis is active or not. CNV, if not treated, may lead to subretinal fibrosis (scarring), a further complication, which is more difficult to treat, and which leads to poor vision.

Good monitoring for patients with PIC is therefore very important.