Cobblestone cannot be converted to Sculk (enable using “Experimental”) using a Sculk Catalyst. I was trying to make a Sculk farm in creative like Mumbo Jumbo did, but I figured out that cobblestone can only be changed to Sculk in Java Edition’s 1.19_experimental_snapshot_1. 

How to reproduce
1: Build a contraption like this with stone like this and put many zombies in it:

2: Watch the poor zombies die as you tell them “it’s for testing purposes”

3: Notice how only grass gets turned to Sculk blocks, but not the cobblestone, it just gets a layer on top of it:

Observed Results
Cobblestone isn’t turned to Sculk like it should be

Expected Behavior
Cobblestone should be turned to Sculk