Not sure if this has been spotted before, but materials other than grass blocks can have farms on top if you drag the initial square from a grass block edge (one of the vertical faces):
As you can see the wheat is growing on a wood floor, the corn on a stone floor and the silkweed on natural stone 
Steps to reproduce:
-Excavate the grass with the voxel mining tool
-Drag a farm zone from the vertical face of an exposed grass block
Although itās not a bug, Itās definitely something that shouldnāt be possible, at least logically speaking.
Cough Cough trees being planted on other trees and houses cough cough.
Nice catch!!!
As per discussion here:
http://discourse.stonehearth.net/t/farming-works-under-water/16238/9
I am marking this as Not a Bug unless @Albert disagrees. For not it appears to be unimplemented as opposed to ābrokenā.
But the difference is that farming dirt is ok (even if underwater)
Farming on wood, stone etc, most definitely not intended gameplayā¦
As Albert said in the other thread, farming under water is not neccesarily intended to work, water just doesnāt restrict it from happening. I am pretty sure that is the same issue we have here, stone (and wood, and other ground types) do not prevent farming not due to a bug, but because they have not been coded to do so. Therefore, this is in essence a āfeatureā (only farm on grass) that is not yet developed at all, and not a bug.
If I am incorrect and stone did previously prevent farming, please let me know. Otherwise, unless I hear from @Albert that this is in fact a bug, it will remain here for now.
Fair enough, but it is at least an exploit as it is coded to not work; you have to drag the area from the side edge of a block like dirtā¦
Missed that! That would definitely be a bug! Sorry 
Yes, that is an bug/exploit with all the zoning tools. Itās a limitation of the current tool which needs to be rewritten.