Crystal heart:
Got two errors when playing this one. The first one was with the second Tierramon I fought during the first mission and the second one was during the first fight. I was trying to capture Tierramons at the time of both errors, so maybe that makes a difference. I was fine when I would fight and defeat Tierramons and then when I tried capturing one and prolonging the fight it gave me the error.
Obsidian heart:
No errors so far with this one. I managed to keep my starter (kept the crystal one too) and finished to the end of the demo. I caught 4 Tierramons and was able to switch them out fine. I had a little trouble capturing a few of the Tierramons but nothing really bad; just learned how the moves worked and strategized to capture the ones I wanted. No serious issues here.
I did have a problem with the map though. When selecting to go back to camp or go to the beginning of the area, it just says 40 and does nothing. In fact, I had some weird stuff happen when I tried it a few times. I think it teleported me back to camp and healed one Tierramon and then left the rest alone. I'm guessing it's not supposed to do that and is supposed to take you back to camp and heal you when you press 2 in the map menu.
Were you planning to add potions or something like them like how pokemon has those to heal your Tierramons? I keep having mine dying and was thinking that potions or something (or maybe mechanical parts or repair parts or something since we're talking about stone here) would be good for healing up and not dying so much. If the map worked correctly and took you back to camp, I'd say that would also work, but something to heal in the field is also a good idea.
One other thing I keep running into that's a bit of a problem is that I forget what moves do. If you put in a menu system with a save function aand showing items and other info, you could just put that in the portion for each Tierramon like they do with pokemon and having stats, then moves, etc. Otherwise, if you don't do that, I would suggest having another key function added during battles where you could press say y (since that isn't being used in the demos yet) and then it would explain what each move does (not the types of moves such as attack, defense, magic, special, but what they do with each Tierramon instead, such as Bloodthirsty, which forget what it does for example).
I'll post again after I start the other two starters to let you know about how it goes.