These are some tumbled seamarbles.  The collection pic was sent by avid beach wanderers Kim and Dave who are lucky enough to live in Hawaii near the beach.  Kim's theory on the marbles is: "The elementary school is on the other side of the highway, then nothing but beach on the beach side. There are a few underground drains that flow into the ocean by the school. So, we think, based on the kind of marbles they are, that they are from way back in the day...kids playing marbles on the playground. I only find them certain times when the sand is all washed away and the rock piles are what's left. This past year, it was only one time that those rocks were exposed."  

What wonderful anthropological wrack combing.