Identifying a children's fantasy book series

There's this children's fantasy series I've been trying to find for years from one book I've read in my youth. From what I recall it was a long-ish series (7-8 books?) and I believe that the powers in the books were based on gems, or some kind of stones.

I know the one book I read was either the last or near last book in the series, and the only part of the plot I can vividly remember were incredibly large sea monsters arriving from the ocean/sea to attack some sort of...tower? I believe there were also illustrations of these monsters (though I'm not sure if I'm not just mistaking those illustrations for my imagination).

I think I also recall a segment about a character trying to escape some sort of mine, where there were Underseers/Overseers, though that may not have been in this same book (I don't remember if I only read one or two books of the series, haha). I also remember there being some sort of love story if that's any help (probably not).

Sorry this is so vague, and I hope someone can give me an answer eventually! It's been really bugging me, and I can't seem to find any series like it anywhere.