Hi, I'm glad you posted some updates for this, because I've never seen it before, and it's really terrific!
I love how well you've used the palette (and even if you could change it, the restricted colours do seem to be challenging your creativity - and you are winning!).
I do like the rock tiles over the ice, if you can only have one or the other, but when I saw the ice, I assumed it would be from a second area.
If this underground world has relics in it, suggesting a lost civilisation, then it would be cool to have squares of cut stone, and leftover bits of wooden structures to also 'break up' the rock tiles. Maybe carved faces, also?
I think the cyan outline looks fine, but if it isn't subtle enough, how hard would it be to make it fade in and out like a soft glow (stepping through darker colours, rather than an alpha, I assume)?
I've been staring at the reflection animation for your crystals with unblinking appreciation - it's amazing! And is that a little page to fit into the diary - does it have game hints and story-advancing clues? I wouldn't be able to wait to collect them, even over treasure!
Are you still going to have coins? I think a stamped image in the center like an eagle or a fish would help them look flat (especially if they had a raised edge).
Is this going to be a mockup, or a working game? (looks pretty convincing atm!)
I like the idea of the alternate layer you showed in
this post, which really gave a suggestion of depth and intricacy.
This not only looks really cool, it's also a great concept overall - I can't wait to see more!