Never seem to spawn. I understand at later times in the server uptime it's possible they could have been overpicked, but from countless checkings for many weeks every single one I have encountered have 0 crystals.
They glow green, and when clicked upon they "Open", but no crystal appears harvestable. After that the glow fades as if it had been harvested.
Not sure if it's bug, feature, or what have you but seemed appropriate to post.
I've gotten tons of them before buddy.
A handful of gather-able objects like this have unknown spawn times. They can be empty immediately after a reset and it's a matter of luck as to who gets it. Of course you will get more of them the more you check them and look.