Probably what is happening is that as you level and place more into luck, you get mre of the lightweight gems.  You are then able to dig more times per trip.  Your shovels aren't breaking more, you are just taking less trips.

Well said, TeT.  As far as I've noticed, nothing affects the odds of breaking a shovel on any given dig click.

The good news is that if you're digging on TA you'll need fewer and fewer shovels as your luck increases.  While you're digging up more and more light-weight stuff, you're also digging up more and more oreputty at 0.2 kgs each.  This means the number of dig clicks required to get a full load of the good stuff is pretty low, and just keeps getting lower.  And as the number of dig clicks goes down, so does the required number of shovels.

Prospector digs up 3-4 oreputty per click (0.6-0.8 kgs), and that (plus the other digable goodies) adds up to a full load after less than 100 clicks.  As a result, it's very rare for Prospector to break more than a couple of shovels per trip.

Sandy

ya, i currently take about 250 clicks per dig trip on TA, and require on average, about 5 shovels (i take 5 with me... about half the time i break all my shovels, and about half the time, i end up having to dump at least one shovel at the end).  this has definitely "improved" since i first went to TA around 60 luck (currently at 100).  when i first went there, there was no forest, so i guess that's not really relevant.  but when i first got to dig in forest, it would take me 7-10 shovels (on average) per dig trip... around 70-75 luck.
--jason