[blattimwind]

Traditional crystals come in what's referred to as a "hermetic metal can", i.e. a metal box soldered closed with leads going through glass seals. I don't know if that's good enough for Helium.

The sealing is mostly because of humidity. I wouldn't expect quartz crytals to be overly sensitive to some gas, since they're basically tuning forks; the crystal physically vibrates in a resonance mode caused by and inducing an electric current across the crystal.