The picture above is of a metallic foreign body in the cornea. Fluorescein dye was placed in the eye so the tears and any abrasions or irregularities glow.
When metal embeds in the cornea, it immediately begins to rust. Within an hour a rust ring will form. So when the doctor takes the foreign body out, a rust ring remains. This also needs to be removed using either a 35 gauge needle or a special “brush”.
A defect remains. If the defect is close enough to the visual axis, anti-inflammatory drops should be used in addition to antibiotic drops to reduce scarring. These heal pretty fast.