Gathering brings in the raw goods. Refining makes the common materials from them, and refining a common one again with an additive makes the uncommon ones. Production takes them from there.
Common leather, refined from Animal Hide and Leather Scrap by Tanning at level 1 (recipe).
One Leather takes 1 Animal Hide and 0.5 Curing Salt.
Uncommon versions: Hardened Leather and Supple Leather
Refined further into:
Production takes it, or its forms, in 110 Tailoring, 34 Blacksmithing, 2 Alchemy, and 19 Carpentry recipes.
What it adds to gear parts made from it: