While clearing out river rocks form garden, son and I found a ton of these little gems!

What are these? Do they just get mixed in with our river rocks? Over time, do they get sanded and polished from the sandy dirt? Perhaps someone else’s treasure they scattered around?

My four year old is in love and can’t wait to go back out in the yard tomorrow to dig for more. I just wish I knew information about them so I can pass it along.

Thanks!