I am trying to track down a science fiction story that I read in the mid-1970s and vaguely remember. This was a short story that was in a compilation book in my school library. The details that I remember are at the beginning of the story. There is a small velvet box with a gem fixed inside of it (possibly an opal or pearl). I believe that there are two characters, a brother and a sister. There is some interaction where the characters are sitting on a park bench examining the velvet box. I believe that the box is used for either time travel or transportation. There may also be a mysterious stranger that guides the characters through the use of the box. I don’t have much more recollection of the story, but please feel free to ask any clarifying questions and I’ll try to assist. Much appreciated!
Could it be the book Escape to Witch Mountain?
Mimsy Were The Borogoves by Lewis Padgett.
http://jabberwockland.blogspot.com/2007/03/mimsy-were-borogoves-by-lewis-padgett.html