Just wrote a response that vanished, so trying again. It’s emery powder, which used to be inside the strawberry attached to a tomato pincushion.
I bought pins some years ago from Harriet Hargrave, and they sent an enormous emery strawberry. It sharpens impressively, even the points of fine safety pins. You can buy it online if you’re interested in making pincushions.
hugs,
charlotte