諸橋勇樹 / アートギャラリー石
Asking the artist about how they were made, he said (basically) that first a layer of charcoal was laid down and then parts were drawn on with pencil (which simultaneously scrapped off the charcoal in that spot).
[Note: Regarding the artist's name - I *think* it's pronounced "Morohashi Yuki", but I forgot to confirm that. In any case, his name is written: 諸橋勇樹
Lyle (Hiroshi) Saxon