Grist for the Mill

From the introduction:

The old post-mill lay idle for years, its runner stone cracked, and the windmill replaced by a new water mill. A month ago, Atin Shaban arrived with a 'new' stone and said he would set the mill grinding again. He repaired the mill-works and winched the new runner stone into place. Passersby noted that the millstone appeared odd – black, not of usual buhrstone, with intricate carvings where the lands and furrows should have been.