Seriously, it was just hanging out in a crate, in the ruined fortress. After that I stumbled into two more of the damn things in a Trolkin outpost.
Crystals outside of a dungeon?????
True that. I legitimately found 28 crystals so far and still haven't gone into the demon dungeon yet, nor have I gone to the ruins in the west. Some have been from the three other dungeons, but more were found outside.
Also I haven't seen anyone mention this, but
do not go into Stormhammer dungeon. Or save before you do. If you try to leave the dungeon the game doesn't know what to do and it takes you to the main menu screen. Its a cool dungeon compared to the others I guess so maybe check it out, just be sure to save before entering so you can reload that save when you can't get out. This was an issue in last revision and I checked this one and it still persists.
Haven't gone to the ruined castle yet either and there are many crates there with more crystals I'm sure. I found some outside of the dungeons in last revision, but not nearly as much as I'm finding now. Also the mazes have a chest at the center that usually has a crystal.
And finally, there are many tall rock formations scattered throughout the map where you can gather charcoal around them. Some of these have a chest on top of them which is really hard to get to and usually requires high jumping skill. Not all of them will have a chest, usually the taller ones, but the ones near the tower have one on each of them iirc. I didn't find any crystals in these in last revision, but I'm willing to bet there are in this version.