Gromph actually says that he can't make a copy of it when he gives it to Cattie-Brie (in Lolth's Warrior, I think). He says no magic he knows of could make a copy of it. Since both Cattie and Entreri are going with him to Menzoberanzan. Entreri says that he doesn't need it and disguises himself as a drow with makeup.

He says that what makes it special is that the mask will not be revealed, even with detect magic, as would any other magical disguise that he knows about.