I was wondering what the common cause of cracked/shattered pocket watch jewels might be? Dropping, temperature, etc?

Wayne perhaps someone tightened down the plate and the pivot was on the edge of the jewel and broke it when tightened down.

Wayne,

Kevin's reply will cover more than half of broken plate jewels for sure! I would also suspect that dropping a watch, especially when fully wound, could cause some breakage. Don't forget a heavy hand can cause center and 4th wheel jewel damage when installing the hands.

Ultrasonic cleaning of non-synthetic jewels has been known to cause fractures. I also believe some jewels may have preloaded stress applied to them after installation that may eventually cause them to break.

I can't stress this point enough; when cleaning a watch, always remove the balance jewels and inspect them for damage. Even a small fracture can affect the overall performance. The oil between the cap and hole jewel can also deplete through these tiny fractures.