Apart from big mines and backpacks, Legifer will also blow crystals to pieces. So will a wand of lightning, striking was already mentioned, Oily Orpiv will work as well, if the crystal is next to a wall or other target. Same goes for fireballs. A big mine is not necessary, a small one will do the trick. None of the other wands or items have an effect as far as I can tell, apart from the pickaxe of course. Silva won't do it.
For kicking, LStr doesn't seem to do much (but you still need a certain minimum, I guess.) Boots, and strong ones at that, are necessary. I finally managed to kick the shard with 1000 LStr and valpurium boots of kicking +5.
Also, this is what UT4 looked like in the end:
Summary:
Things that break crystals:
- Mines
- Big mines
- Backpacks full of Gunpowder
- Wand of Lightning
- Wand of Striking
- Wand of Fireballs
- Oily Orpiv
- Legifer
- Pickaxe
- Kicking, if your boots are strong enough
Edit - did I really type backpax?