[JMS] How does one "grind a crystal" to change its frequency ?

[Sherrill Watkins]

Friends and Members of the James Millen Society: Please be advised that from
time to time, I see FT-243 crystals for sale in odd frequencies; i.e. that
are outside of the amateur bands and of no use. Years ago, I heard that it
was possible to "grind" a crystal to raise its frequency. Also, I heard that
one could "rub" a crystal with pensel graphite to lower its frequency.  Yet,
I have never seen any article published in any major amateur publication
including various handbooks, or magazines such as QST, Electric Radio, The
Old Timers Bulletin, etc. as to how this process is be performed.  Can anyone
refer me to a good source or explain the process in great detail? Also, what
is meant by the term, "overtone" crystal? Thank you very much. - Sherrill W.
k4own, member