Googling it didn't seem to lend any results except the unrelated killing two birds, etc, stuff.
Edit: In Berkshire Hathaway's 1995 annual meeting at 1:04:54 (or second occurrence of the word "stone" if you search page text on the transcript and click on the phrase to jump to the timestamp. Charlie says in response to an audience member regarding "Warren changing over the years" "I'd say about one stone", which is meant as a joke. I guess, like the first answer says, it probably jokingly mentions weight or something like that.