1 mm= .039 inch. if you have calipers, they can give you an idea of what that size is. Very generally speaking, standard paper is about .003 thick. So 12 pieces of paper together may give you an idea of 1mm thickness. Regardless of grain size, I don't think 25 lb would barely cover the bottom of a standard 75.
Hope the red garnet shimmers like nerds candy!!! lol