Why some people have dark rings around their iris and others dont have?

Im wondering why some people have dark rings around their iris and others dont have it.
It looks really cool & beautiful. Many contact lenses nowadays have dark outline around the iris. SOme people are lucky to have it naturally

Picture of a person with dark rings around their iris:

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No dark outline iris:

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I noticed that some people have thicker dark rings and some have thin dark ring around the iris.
Iris with thick dark outline is rare.
Is it dominant or a recessive trait?
Most brown/black eyes people dont have the ring too.

I have thick dark circles around my irises. I didn't know it was rare. I figured it was genetics.

I have dark brown eyes with a black ring around it, im half turkish