there is 1 rock fragment for each excavation site.
as you know, they are hiding in some rocks outside the site.
if you can't step on it after rock smashing the rock, there is rock fragment.
then you press Space Bar to pick it up, it may be that there is no any dialog when you pick it up.
but if you can step on it after that, it means that you have already pick the rock fragment up.
then you can move on to another site and do the same thing.
remember that there is 1 rock fragment for each excavation site, so there are 6 of them.
Thx for your help