Here's some things, that I can remember from the top of my head for you to check. To make it easier to understand, lets call the new material 'gold':
materials.xml - define the 'gold' material
items.xml - create two items, one 'gold', that has the 'gold' material set as its material and another would be unit_gold item (the item must be named unit_ + MaterialName = unit_gold), that would: