The acid mentioned in a earlier posting was Hydrofluoric acid, a very
dangerous acid, not because of its strength but of the way it attacks the
skin. The use of this is not to be undertaken likely, despite the fact I
believe it was used for cleaning stone buildings of grime. I guess that was
before all the Heath and Safety legislation.

With sealed crystal units the final trim is usually based on the weight of
metal electrode evaporated in vacuum onto the quartz surface. Some change in
frequency can be made bt thickening or reducing the electode....electrolytic
plating of copper on was suggested at one time, though I have not tried
it....the use of gold plating solutions is also highly dangerous !!  It is
probably cheaper in peace of mind if nothing else to shell out for a custom
rock, because any of these processes with compromise the stability and aging
rate of the unit.

With old FT-243 and 10X holders I recollect that a proprietary brand of
abrasive cleaner was found to be quite effective (Vim or Ajax in the UK ).
All but the finest grades of carborundem (emery ..Aluminium oxide  is
better) can result in the blank quickly losing parallelism and ceasing to
oscillate.

For larger movements it is better to lap several blanks of the same start
frequency simultaneously by "wringing" them down on a glass plate with hot
candle wax and then lapping them against another piece of plate glass. It is
possible to move them 500kHz without losing activity this way.

Great fun, but quite frustrating sometimes as the desired rock would lose
activity just as you got it near to the required frequency !

Cheers de Alan G3NYK