Some people have very dark blue eyes, almost appearing black in certain lighting. Blue eyes are caused by low levels of melanin in the iris, but just how dark they appear depends on additional factors. The amount of melanocyte cells, thickness of the stroma, and light scattering all influence eye color. So why do some people end up with such dark navy blue eyes? Let’s explore the genetics and anatomy behind very dark blue eyes.
Melanin Concentration
The main determinant of eye color is the amount of melanin present in the iris. Melanin is a pigment produced by cells called melanocytes. It comes in two forms:
| Eumelanin | Brown/black melanin |
| Pheomelanin | Red/yellow melanin |
People with blue eyes have low levels of melanin overall. But even among people with blue eyes, there is variation in melanin content. Those with very dark blue eyes likely have more eumelanin than individuals with light blue eyes. However, the concentration of melanin alone doesn’t fully account for why some blue eyes appear so dark.
Melanocyte Cell Density
In addition to the concentration of melanin, the density of melanocytes also impacts eye color. Melanocytes are cells that produce and store melanin. Even with the same amount of melanin, eyes with more melanocytes spaced closer together will appear darker.
Research has found those with darker blue eyes tend to have higher melanocyte cell counts per area compared to those with light blue eyes. Their melanocytes are more densely packed with melanin granules as well. Higher melanocyte density results in a darker overall eye color.
Iris Stroma Structure
The stroma is the layer underneath the anterior border cells of the iris. It consists of collagen fibers, blood vessels, pigment cells, and other structures. The thickness and composition of the stroma affects how light scatters and absorbs in the eye.
People with thicker stroma tend to have darker colored eyes. The increased density of the collagen fibers causes less light to reflect out, creating a darker hue. Dark blue eyes likely have a thicker stromal layer compared to light blue eyes, contributing to their darker pigmentation.
Rayleigh Scattering
Light scattering by particles in the stroma also determines eye color. This scattering, known as Rayleigh scattering, causes shorter wavelengths of light to diffuse more.
In people with low melanin, this results in more blue light being reflected out while other wavelengths are absorbed. But in those with higher melanin and dense stroma, Rayleigh scattering is substantially reduced. This causes the eyes to appear very dark blue or even black in some lighting.
Genetic Factors
Genetics play a major role in determining melanin levels and density of melanocytes. The key genes involved are:
| OCA2 | Codes for protein that controls melanin production |
| HERC2 | Regulates OCA2 expression |
| SLC45A2 | Involved in melanin synthesis |
Certain variations in these genes are associated with reduced melanin production and lighter eye colors. However, even for blue eyes, there are genetic differences that result in different shades. Those with very dark navy blue eyes likely have genetic variants that still limit melanin synthesis relative to brown eyes, but allow for higher melanocyte density and melanin content compared to other blue eyes.
Ethnic Background
Ethnicity also seems to play a role in eye color variation beyond simple genetics. For example, people of European descent with blue eyes tend to have lighter coloration than blue-eyed people of Asian ancestry. This may be related to differences in stromal density and cell types that have yet to be fully defined genetically. Environmental factors influencing development may also contribute to subtle distinctions between ethnic groups.
Sun Exposure
Interestingly, some research indicates that sun exposure impacts how dark blue eyes appear later in life. Chronic sun exposure seems to stimulate an increase in melanin production over time. This results in a gradual darkening of the eyes that is more prominent in people starting with light eye colors. So childhood environment may explain part of the natural variation in blue eye shades.
Diet
Diet and nutrition during developmental years may potentially influence eye color as well, though more research is needed. Carotenoid pigments found in many fruits and vegetables can accumulate in the skin and eyes. This may in turn modify melanin absorption and distribution. However, any diet-related impact on eye color is considered minor at best.
Eye Injuries
In some cases, eye trauma resulting in heterochromia can cause one eye to appear darker. The injured eye can develop increased melanin over time, darkening the iris independent of the other eye. Differences in melanin levels between eyes is most noticeable in people with light eye colors to begin with.
Conclusion
In summary, dark navy blue eyes are the result of multiple factors. While low melanin levels are responsible for blue eyes in general, higher melanin content and density of melanocytes lead to darker shades. A thicker stromal layer and reduced Rayleigh scattering also contribute to very dark blue eye coloration. Genetics play a major role, but development, environment, and ethnicity impart subtle differences in shade as well. So if you have very dark blue eyes, that’s the complex anatomical and genetic recipe behind your rare eye color.
