Stupid mining questions

[Collock]

So I have been going on the assumtion when surveying a cave for vains. If I get nothing then it is the same as a rock. Is this true or due I need to mine until I get rock to be sure there is no vain in a square?

[Lumar]

mine till you get drock to drop then move on if it does. keep mining nearby tiles if it gives ore or nothing. sometimes ore veins fuck you and give no ore every tick