sure it is not a grade sword, it is just some sword. Queen is not a swordsman.
that applies for King's sword as well , it has some mechanism which mean it is not really a grade sword either.

A haki clad sword makes it virtually unbreakable, not even a 2 Yonkou combo attack could break Sandai, Mihawk wouldn't have broken pts Zoro swords but Zoro cut through King's strongest attack and coated sword, why? You know the answer and it's not some funny blue light. :myman:
Sure, it is looking like a haki just like how Sanji fans say Sanji used haki by showing very obscure black shading panels. Good that trend is followed.