I have a grammar which says that:
"The 'noun+noun' structure is normally used to say what things are made of."
"A few pairs of nouns and adjectives are used as modifiers with different meanings. Generally the noun simply names the material something is made of, while the adjective has a more metaphorical meaning."
a gold watch - golden memories;
a silk stocking - silken skin
I've also heard that the "-en" ending is used in a poetic sense. But when I looked up at my dictionary for the word "wooden", it brought as an example "wooden bench"; even though "wooden" wasn't being used in a figurative nor in a poetic way. Furthermore, I don't know whether to use "wood door" or "wooden door", meaning that the door is made of wood.