Girl and her brother find a crystal (possibly in a pond) that lets them travel to another world

[Emlyn]

This is a book I read as a child, so about 20-30 years ago, I don't think I ever finished it but it was from my school library and I never found another copy to read later. I can't remember too much of the plot, but I think a girl and her brother get sent to relatives house for the summer, while exploring the area I think they find a wooded area with a pond and in the pond is a crystal? I'm fairly sure that this crystal lets them travel to another world but I can't remember what that world is like! It had a greeny/turquoisey cover.

I probably won't enjoy it as much as an adult now, but it's a book I've always wanted to go back and read fully, as desite not remembering much of it, it's always stuck in my mind. Any help figuring out the title would be greatly appreciated, thank you

[Sirvantes]

Probably not this, but the first book that came to my mind was The Magician's Nephew by C.S. Lewis. Just a guess because of the similar touchpoints: boy and girl, relative (uncle), exploring the area, woods, water (pools), travel to strange worlds, gems/crystals, although these are set in rings.

[Anne]

Ruth Nichols wrote two books. A Walk Out of the World, and The Marrow of the World. Both are kids’ books that feature two kids, boy & girl, who get drawn into another world. I’m looking at my copies right now.. it’s been a long time! They sound like good possibilities! I sense a reread for me!

[Ayshe]

There's also Spilling the Magic but I'm not certain it has the crystal detail.

[Josie]

There's one called the Diamond in the Window.
https://www.goodreads.com/book/show/1...

[Capn]

Anne wrote: "Ruth Nichols wrote two books. A Walk Out of the World, and The Marrow of the World. Both are kids’ books that feature two kids, boy & girl, who get drawn into another world. I’m looking at my copie..."

I think we can rule out A Walk Out of the World (just read it this year). It's good, though. :) (I sure would like to read The Marrow of the World!)

Perhaps The Silver Nutmeg: The Story of Anna Lavinia and Toby? (It's a to-read for me - someone who has read it may rule it out for you). :)

The Shades has a dolphin fountain, but it comes up occasionally (and is sadly very very scarce now). The Diamond in the Window, as Josie mentioned just above, also pops up often. :)

William and Mary has a greenish cover too, and crops up now and again. Seashell in that one.

We have a rather short list (feel free to bulk it up!) on Fiction: Portal is in Water