An iron line of the cornea was detected in 26 of 48 eyes that had undergone filtering surgery for glaucoma. The lines were located in the corneal epithelium immediately anterior to the bleb. In 13 eyes the "bleb" was flat, and in none of these cases did an iron line develop. It seems likely that the iron line results from the minute, repeated, localized trauma caused by the eyelid striking the elevated bleb.
Ferry AP. A "New" Iron Line of the Superficial CorneaOccurrence in Patients With Filtering Blebs. Arch Ophthalmol. 1968;79(2):142-145. doi:10.1001/archopht.1968.03850040144005