ABANDONED. YA Sci-Fi Late 1980's - Child finds alien stone/pebble on beach

[Muse79]

All I can remember is that a child (possibly a boy) was collecting pebbles on a beach and found a strange stone and kept it. I think he showed the stone to a girl (possibly his sister) and the stone did something and they thought it was magical. Then they were followed and chased by people or agents that wanted the stone. I think it was an alien device or something. I only read part of it before I had to return it.

[Ann aka Iftcan]

Black Suits from Outer Space possibly? Not found on a beach, but the rest of it kind of fits. There are 2 more books in this series, so possibly one of them could be the specific one you are looking for.

[Lobstergirl, au gratin]

Muse79, are you still looking for this or did you find it?

[Etta.n]

Is one of the antagonists his possible-future self? The Green Futures of Tycho possibly if so

[Lobstergirl, au gratin]

Lobstergirl wrote: "Muse79, are you still looking for this or did you find it?"

No response; moving to Abandoned.