Sekkaku (the stone core) (石核)
Sekkaku (the stone core) refers to the ore that remains after tearing off the flakes, which usually serve as the raw materials for chipped stone tools. Strictly speaking, it is divided into the ore, from which the flakes are peeled off, and the residues, which are not used and are thrown away after a series of flake-exfoliation processes. In this case, the former is called sekkaku and the latter is called zanshi. There are trimmed and untrimmed sekkaku.
The appearance greatly varies by time and place. And it provides a big clue in verifying the fabrication methods of chipped stone tools.
The photograph is that of the sekkaku of kokuyoseki (obsidian).