A very hard black solid consisting of silicon carbide, used as an abrasive.
Otherwise known as the stuff they make knife-sharpening stones out of.
The walls of Zodanga are seventy-five feet in height and fifty feet thick. They are built of enormous blocks of carborundum, and the task of entering the city seemed, to my escort of green warriors, an impossibility.
I don’t know whether it’s a particularly good thing to build walls out of, but it’s not like I know all that much about wall-building.
Then again, Wikipedia says you can make bullet-proof vests out of it.
Definition via Kindle (The New Oxford American Dictionary)