quoting the wiki: "In each Excavation site, there is also one special breakable object, which can be identified by shiny sparkles floating around it. These shiny breakable, on top of the regular Pokémons and items, also have unique Pokémons that can spawn. Furthermore, Pokémons do not run away even if the player does not have enough Discovery points."
The wiki is probably outdated as shiny rocks dont have unique pokemon anymore.