[It certainly is a stubborn substance, this powder, but she wants as pure a sample as possible, so she doesn't resort to water, instead continuing to rub with a focused determination, tongue sticking out the side of her mouth as she keeps working.]

Well, it seems to serve the same purpose as makeup, in a sense, but... I have never seen a makeup this good at sticking to such a surface, I am not sure you could purchase it.

[The makeup she's familiar with would surely leave residue or ruboff on his clothing, smear where garments were worn...

But she doubles her efforts and keeps it up until there is a small circle of black exposed, after which she encourages him to turn his arm slightly and allow her to rub an equally small spot on the exact opposite side of the one she's already made.]