Advice on puzzles/riddles requested

I've begun work on a MG fantasy and would like some advice for a specific part.

The MCs are a twin brother and sister, Edmund and Caroline Grant. Edmund's keen on medieval things and fantasy. Caroline's keen on math and science, especially astronomy. (One of their parents - maybe both - is an astronomer, and Edmund and Caroline were even named after major astronomers: Edmund Halley of Halley's Comet fame, and Caroline Herschel, the sister of William Herschel.) They get into recurring arguments about each others' interests; Edmund thinks that Caroline is stuffy and unimaginative, while Caroline thinks that Edmund is an impractical daydreamer.

In the story, they're looking for a long-lost object (I was thinking of a large jewel, a bit like the Arkenstone in "The Hobbit"), and to find it, they have to solve a puzzle or riddle (or more than one). And solving the puzzle(s) requires a mixture of myth and medieval lore (Edmund's area) with mathematics and science, especially astronomical science (Caroline's area), which means that they need to combine their skills to find the jewel.

Does anybody have any ideas to get me started on what the puzzle(s) might be?

Debbie V said:

Without living in the world of your book it would be hard for anyone to come up with this. You may want to work backward. Where is the stone? Research the topics the kids are interested in so you are more expert than them. Once you have those two things clear, decide what obstacles they'll face. Put the three items together so solving the riddles will move your characters into and out of the obstacles.

Good luck.

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Thanks. Good point, Debbie, and I'll use the comments you raised to help me with that.

Incidentally, alleycat, why did you delete your post?