To make obsidian adamantine-level, copy most of it's numbered properties (solid_density, yields, fractures, etc) on the material definition file.
Upon further testing, I've begun to have remarkable trouble milking any custom creature. I've even tried directly copying and pasting a cow's milk entry into a similar creature, and it doesn't even work then. Is milk ridiculously hardcoded or something?
EDIT: I've also noticed that in the default raws, something weird seems to be going on. Milk uses a [USE_MATERIAL_TEMPLATE] in the female caste, even though that's technically a creature token (I haven't been able to get it to work in castes). What sorcery is this?