The Giant Hrungnir used a whetstone as a weapon against Thor.  While able to shatter it with Mjöllnir, a piece of the whetstone is lodged in Thor's forehead.  Seems like whetstones might have been viewed as sacred to Thor --  they were used to sharpen weapons for battle too.

--- On Wed, 4/15/09, Fred and Grace Hatton <hatton@...> wrote:
From: Fred and Grace Hatton <hatton@...>
Subject: [norse_course] whetstone
To: norse_course@yahoogroups.com
Date: Wednesday, April 15, 2009, 7:09 AM

Just a guess, but the whetstone could break and that would not be a good
thing. They depended on them to sharpen all their weapons and every day
tools.
The type of stone suitable for a whetstone may not have been available in
the area and perhaps needed to be imported?
Grace
Fred and Grace Hatton
Hawley Pa