I have ground 6 MHz crystals up 1 MHz - just to see if I could do it. I
grind in a figure 8 pattern using fine sanding paper. I often use 600 grit
to speed things along.

I use a small square piece of glass I got from the garbage can of the glass
cutter at the local hardware store.

Clean well after grinding.

It took me one NBA basketball game and two cold beers to go up one MHz. If
you secure the glass plate, you can grind with one hand and drink with the
other.

I now have a large box of ham crystals - even made a nice label for it.

73, Colin K7FM