Think I've worked it out, not confirmed yet. Fire bombs clear some rocks... interesting.
Edit: Confirmed. I hope if anyone else is stuck doing this and frantically googling, that you find my post. If not... well... I got the thing, so... yay =D.

As you doubtlessly discovered, there's a note in Goldhaven where the last Mining Certificate is, which mentions the reinforcement of those walls, with a single section not having a metal lattice. It follows that this section is weak and thus can be mined, then sapped through in order to get to that chest; a skill you pick up in the Gem Mines.

Indeed, I didn't know you could use fire bombs to clear rocks. I found myself just lobbing those all over the place and found a whole ton of things. Was great! I'll need to replay this some day, was a great time.

No argument here! I'm giving it another attempt myself, with the aim of 100% it within 12 hours if I can for a YouTube playthrough (done Lost City already in around 5).

Heh, and I was the same with those mounds of stones, you know. I thought you had to dig them up, but it turned out they had to be mined. Loads of great loot totally missed during my earlier goes, I can tell you now.

I was watching Alex Diener play it during my last playthrough. (He gets really angry at people trying to help him, it's mega cringe, like, dude, chill)
Will watch someone else next time. Hopefully someone more chill. XD
I enjoy multitasking while playing games, and it was super fun to watch someone play the same thing I was doing.

And yeah, replaying it I'll randomly remember things and be like "Woah, yeah, I can get this thing early!" and be super OP early. XD

Well, I can certainly understand people wanting to do it for themselves, and I've broke plenty of controllers in frustration myself over the decades. For the most part though, I've always treated gaming the same way many people treat sports, as a challenge to overcome; me against the programmer as it were.

You're not wrong in what you say at the end there either. Discovering you could get the Ancient Spear from Siren Deeps extremely early on in Final Adventure, not to mention the Icefall Hammer as well (make no mistake, it's not easy, but perfectly doable) completely changed the way I play that mod now.

That's fair, but he was getting properly mad at his viewers, which just comes off as childish. Especially as he was having a terrible time in the previous video.
Getting mad at people, who are trying to help you. When you haven't told them that you don't like people trying to help you, is massively dickish.

Indeed, as well as having that extra foresight when it comes to how you build your characters ^_^

Not sure if I ever watched his playthroughs specifically myself, but I do remember another guy getting well stressed at vanilla Grimrock II when it came out and ignoring all advice, good or bad. I don't believe he ever finished it either, unable to deal with all the spiders in the Crystal Mines.

Hmm, and I usually stick with the default party for the extra challenge. Just made Goldhaven in less than 4 hours with nearly half the secrets, so perhaps it is possible to do it within 12 hours after all. I'll keep at it anyway.

The spiders are pretty awful. Can't fault them there.
I hate the big ones, and the little ones that you can't kill. I have to like, focus on a spot above my screen while killing them. XD

There's a mod for grimrock 1 that replaces them with other random enemies, I think that was pretty great. Be nice if there was a mod that'd change them all into something else, even ones in mods. But that'd be pretty difficult to do.

His playthroughs are on youtube, and while he's for the most part fun. His later videos are stressful as he starts playing really badly and then getting really really angry at anyone who tries to help... but then getting really angry at the mod again. Not very fun content to watch.