Unless being covered in impossible-to-clean metal shavings is your idea of a good time, leave this experiment to the professionals.
The vast majority of an Etch A Sketch is plain old aluminium powder, ground extremely fine. It’s what coats the glass screen of the toy and gives it that distinctive grey color. The only other major component is a metal pointer, the position of which is controlled by a couple of strings attached to the knobs on the faceplate. Moving the knobs (and the pointer, by extension) scrapes the aluminum powder away, creating the drawing.
Which reminds me: a shake-to-erase feature would definitely ease my frustrations with Photoshop.