As a sort of touchstone, however, she began to speak of his kindness in conveying the aunt and niece; and though his answer was in the spirit of cutting the matter short, she believed it to indicate only his disinclination to dwell on any kindness of his own.
There are no more uses of "touchstone" in the book.
Show samples from other sources
"I’m not a good touchstone to use for judging your sanity."
Patrick Rothfuss -- The Name of the Wind
I considered Yamacraw a touchstone: if the Yamacraw children knew about it, then the chances were excellent that the vast majority of American children had been reached.