Why do rapiers switch from GSM to BSM at level 70?

I got a nice rapier drop from the Ala Mhigo level 70 dungeon. I equipped it and after a couple of fights went to repair it...but that wasn't an option. Turns out instead if being repairable by a Level 60 Goldsmith like all of the other rapiers I'd used up to that point, all of a sudden, it needs a level 60 Blacksmith. Eventually I'll get those last three levels of BSM, but I'm curious about the reason for the switch.

its random, there are some weapons that switch between bsm and goldsmith

I'm fairly certain a blacksmith would be more likely to make a rapier in the first place.

I'm going to hazard a guess and say it's due to what the weapon is meant to be (lore wise) made from - it's probably made from stone rather than metal. Several GLA/PLD swords are like this, the machaulati type swords are actually made from wood and thus are CRP repairable, and the PLD relic weapon Curtana is canon wise made from a solid slab of marble (this is confirmed by the designer in the artbook), and thus is repairable by GSM instead of BSM. Most likely the RDM rapiers are in the same boat here.