Found: Sci-fi fiction book about alien tournament looking for a stone

I read this book probably around 19 or more years ago. Details are all fuzzy but… It’s a fiction book about the character at a summer home where there is an island off in the distance. They look at it through a window and they realize that there are marks in the window that correspond with the island. I think they go to the island and find a special stone. They meet other characters along the way who offer to trade this special stone with potions that can either make them incredibly strong, smart or something else. These other characters are actually aliens all in search of this special stone. The main character does not trade with either of them and she becomes part of this competition. One of the characters turns into this arachnid like creature and that’s all that I can remember. I can’t even remember details about the main character. It’s been a long time since I’ve read this book. Also I don’t remember if it’s a stone that the main character found. All I remember was it was something that these other intergalactic beings all wanted or needed.

I think you're thinking of Interstellar Pig by William Sleator. Which is definitely fiction.

Yes thank you, it was a typo putting non fiction. And you are correct, that is the book. It’s been a long time since I’ve read it and the details were fuzzy. It popped into my head when my son was looking for a book to read and I just couldn’t get it out of my head. Thank you for your help.

You're welcome! I still have a copy of that book- read it when I was a teen and thought it was great.