Time for me to add my tiny bit of semi related, but useless, knowledge : Galena can also form in cube shapes.
The only reason I know this is because we were crawling around in some old mines in the Ozarks and back in a little offshoot I found a few small, dark cubes in the dirt so I asked some of the geologists with us. Turns out the small mines were there specifically to pull out galena.
From wikipedia :
Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver.

Also, don't go crawling around in mines from the 1800s, it's likely a bad idea.
I'm starting to feel like a silverbullet.md shill because I post it so often these days...