Sheep crust

Crust leather is the term applied to leather, which is dried after tanning but has not yet been dyed. In the case of chrome-tanned leather, this is known as "chrome crust" and, for vegetable-tanned leather, as "vegetable crust". Get Quotes. Santosh Pargal & Co.
In other words in 1804 after diplomas ceased to be made of actual parchment (which is made from sheepskin), they were called sheepskins as link to the tradition of making diplomas from parchment. IMO, sheep skins look better on sheep than on our walls!
Besides, crust leather is incredibly soft, pliable, and ages beautifully because of its rich leather patina. It can be used both in its natural state or top-coated with a surface dye to obtain special effects.