330D: 70s (or earlier) science fiction short story featuring a gem in a velvet box used for time travel or teleportation

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!

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