What's the Name of That Book??? discussion

This topic is about
Over Sea, Under Stone
SOLVED: Children's/YA
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SOLVED. YA Fantasy. Magical orbs of perhaps different mana, seaside cave? [s]
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When I was about 14 (2000?) I read in the library a book that had different orbs or balls that had magical properties. Maybe there's a fire one, and a water one, etc.
There was a main kid, who i think was a girl, but there were other characters.
I recall one scene in which she and a friend are searching down in a seaside cave. The tide is coming in, and they are looking for something.
I though it was a wrinkle in time, but now I don't think it is.
If I recall it is NOT some grand fantasy world LOTR style with orcs or dragons. It's in our world. There are places in our world that are connected to the other world or power of some sort, so the physical location is important. But it's def more of a back in the day sort of vibe, no computers or not even a phone I don't think.
I wish I could remember more details. I thought it was maybe part of a series? It must be somewhat popular, our small school library did not have that many fantasy books. Thank you in advance for your help.