Sentence:  It was as if a stone were ground to dust; as if white sparks flew from a livid <WSD>whetstone</WSD>, which was his spine; as if the switchback railway, having swooped to the depths, fell, fell, fell. 
Meaning of the word marked with <WSD> tags:  A sharpening stone; a hard stone or piece of synthetically bonded hard minerals that has been formed with at least one flat surface, used to sharpen or hone an edged tool.