I'd Help

[Thomas]

I find these in an orchard I work in every now and then we don't use chemicals  or fertilizer that would create these ,when I find new ones they have black covering them but irrigation  removes it white under black ,I broke one open baby blue inside .just wondering were there coming from magnet does  not stick to it .any help figuring  this out will be appreciated. 

[Steve Herschbach]

Looks like an extremely common volcanic rock.