It's all random. Most of the time you convert an iLvl 55 item you get a Grade 3 Materia related to the stats on your gear, or if you're lucky it'll be a Grade 4 Materia. Your Militia Ringbands will probably become either an Intelligence, Savage Aim, or Heaven's Eye materia; perhaps even an elemental resist, which would be a shame. But as far as knowing just what it'll be? No possible way of knowing.