The Mod ID is literally "atum", as seen on the Block and Item Names (which cannot start with anything other than the right Mod ID)
Do they behave like regular Sand/Stone?
I should really get more into MC modding to not always sound like a n00b when posting such stuff ...
The Stone, yes, when mined it turns into a "cobble" version (atum:tile.cobble). Also every possible decoration object is craftable like from regular stone (bricks, slabs etc.)
The Sand, kind of, has no gravity (because of being more solid than normal Sand and taking longer to shovel).
Oh and something else you might want to know, out of the cobble versions, tools are craftable equivalent to the vanilla stone tools.
IDs og them are:
atum:item.limestonePickaxe
atum:item.limestoneSword
atum:item.limestoneAxe
atum:item.limestoneShovel
atum:item.limestoneHoe
Idk why the dev named the cracked variant of the stone limestone
Extra Info: It's really hard surviving there, because the mobs who spawn there are a lot more stronger than in the overworld. (obviously not saying because hoping for more rare materials to find :P)