A special abrasive made by Gray Blacksmith.
It was made to be suitable for physical Thanos' and Thanos'-AD weapons.
Both refine level and card are preserved.

[Gray weapon list]
Thanos Sword
Thanos Greatsword
Thanos Spear
Thanos Two-Handed Spear
Thanos Dagger
Thanos Katar
Thanos Knuckle
Thanos Hammer
Thanos Axe
Thanos Whip
Thanos Violin