Attaching materia requires the “materia melder” key item, an apparatus which can be obtained via a quest available to Disciples of the Hand level 18 or above (with the exception of culinarians and alchemists). The apparatus allows players to attach one piece of materia per item.

Players who have met certain requirements can obtain the more advanced ”augmented materia melder,” which allows the attachment of up to five pieces of materia per item. Players can never fail to attach the first piece of materia. However, the success rate progressively decreases with each subsequent piece of materia. A failed attempt to attach materia will result in the loss of all items involved in the process, barring the key item.
Hmmm. does that mean the augmented materia melder or the base gear..... It sounds like 5/5 materia gear is going to be very costly.