You can easily add custom materials for the tombstones by simply calling
tombs.register_stones('modname:nodename', 'nodename', 'description', 'texture', #Light_Level)
modname:nodename is the recipe node used in the input slot.
nodename is used internally, keep it short but specific to avoid conflicts.
description shows when hovering over the node in inventory.
DO NOT include the extension for the texture or you will get an error.
#Light_Level is optional if used it should be a number 1-14.
