jun1per

Figured it out! 
There's a block that dictates the knapping surface's texture under "blocktypes/stone/knappingsurface.json". You can patch in whatever textures you need with something like this:
[ { "op": "addmerge", "path": "/textures", "value": { conglomerate: { base: "block/stone/rock/conglomerate1" }, limestone: { base: "block/stone/rock/limestone1" }, phyllite: { base: "block/stone/rock/phyllite1" }, slate: { base: "block/stone/rock/slate1" }, kimberlite: { base: "block/stone/rock/kimberlite1" }, scoria: { base: "block/stone/rock/scoria1" }, tuff: { base: "block/stone/rock/tuff1" }, bauxite: { base: "block/stone/rock/bauxite1" }, halite: { base: "block/stone/rock/halite1" }, suevite: { base: "block/stone/rock/suevite1" }, whitemarble: { base: "block/stone/rock/whitemarble1" }, redmarble: { base: "block/stone/rock/redmarble1" }, greenmarble: { base: "block/stone/rock/greenmarble1" }, travertine: { base: "block/stone/rock/travertine1" }, coralchunk-dead: { base: "block/creature/aquatic/coral/sponge/dead2" }, }, "file": "game:blocktypes/stone/knappingsurface.json", } ] Hope this helps anyone else who runs into this problem!