what causes a ring around the pupil

The white ring you see on top of your iris is most likely a deposit of cholesterol around the edge of your cornea, the clear dome that covers the iris.

This cholesterol deposit is known officially as a pinguicula. If you are young, this could suggest high cholesterol; if you are a senior citizen, it may not, as it is fairly common in older people and not necessarily suggestive of high cholesterol. The white ring stays around the edges, like a halo, and does not cause damage to your eye or your vision.