These are the refill leads for a Mechanical Chalk Pencil which is packaged with a white lead. They are "ceramic" lead, which means they do not flake off or mark your thread if you stitch through them with white silk, for example. The most popular color is "grey", which more closely imitates a pencil lead, and is useful for marking fabric and canvas. One retailer uses the grey lead to draw stitching lines for darning patterns for open canvaswork.
