The ring is called "arcus senilis" in people say 60 or older. It's cholesterol deposited in one of the layers of the cornea. It's considered an aging change like wrinkles or the hair turning gray.
In people younger than 60 it can be a sign of elevated blood cholesterol and anyone with one younger than say 60-65 should have a fasting lipid profile drawn. I've discovered hundreds of people with unknown high cholesterol by asking them to have it checked when I see an arcus juvenalis.
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