There is a condition called arcus senilis, where a grayish ring forms around the outer edge of the iris (colored part of the eye). This ring is often more pronounced in the top half of the eye. In older adults, arcus senilis is a normal part of eye aging and is never something to be concerned about. In younger adults like you, however, it is important to have a complete check up with your eye doctor
and with your primary care doctor
. This is because, if this does turn out to be arcus senilis, then it may be a sign of high levels of cholesterol in your blood stream.
If your doctor says that this is not arcus senilis then, on the other hand, it is worth noting that as we mature the pigment levels in our irises do change slightly. Therefore, it can certainly be possible to notice changes in the color of your iris, say from brown to gray, and for that to be completely normal.
Finally, rarely, rings of color around the iris may be signs of another more serious medical condition, such as Wilson's disease.
Start by setting up a visit with your eye doctor at your earliest convenience.