Are there crystals that are tolerant of high pressure?
I've used solid ceramic resonators in high-pressure environments (inside oil-filled canisters at 120m below sea-level) without any trouble.
Of course as Majenko has already alluded to they're typically not quite as precise as a crystal usually is, but they quite happily clocked my PICs and were accurate enough to allow my serial comms at a few 100k-baud.

I did consider encapsulating a standard crystal in epoxy as suggested bu others, but my concern was that although many epoxies are generally tougher and harder to break than a crystal, I didn't think they would provide much protection.
My thinking was that the reason epoxy is tough &amp; hard-to-break is that it is able to flex or deform somehow by some small amount before breaking. But this would be transferring force to the crystal - exactly what I was trying to avoid.