Plant- and animal-related item tokens do actually use one more argument than normal. This, for example, is what you'd use to smelt some plump helmets:
[PRODUCT:100:5:PLANT:NONE:PLANT_MAT:MUSHROOM_HELMET_PLUMP:STRUCTURAL]
Change the second-to-last argument to PROCK or whatever ID your rock-plant ends up having. If you have trouble figuring out item tokens for reactions and itemcorpses, the best way to figure it out is to make an embark profile that has that item, and look at the profile in the data/init folder. The embark profiles and reactions use the same syntax for items, which is very useful.
Tried that. Here's the raws for the "plant"
[PLANT:PROCK]
[NAME:processed rock][NAME_PLURAL:processed rocks][ADJ:processed rock]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[MATERIAL_VALUE:1]
[EDIBLE_VERMIN]
[EDIBLE_RAW]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
[PICKED_TILE:127][PICKED_COLOR:0:1:0]
[DRY]
[USE_MATERIAL_TEMPLATE:DRINK:PLANT_ALCOHOL_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:frozen rock brew]
[STATE_NAME_ADJ:LIQUID:rock brew]
[STATE_NAME_ADJ:GAS:boiling rock brew]
[MATERIAL_VALUE:1]
[DISPLAY_COLOR:2:0:0]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[PREFIX:NONE]
[DRINK:LOCAL_PLANT_MAT:DRINK]
[FREQUENCY:0]
[CLUSTERSIZE:5]