Vegetable-tanned leather (also known as Oak tan leather) refers to the tannage or method of tanning the cowhide into leather. It's called “vegetable” because of the natural materials used in the tanning process like tree bark and saps.
Vegetable tanning or "veg tan" as it's sometimes called, is one of the oldest methods of tanning known to man. Veg tan leather has open pores and will build a very rich and deep patina over time.
Each veg tan product will begin light and pale in color, and slowly transition to darker and rich in color after plenty of use.
It's the natural elements in your day to day that will expose your veg tan product to things like sunlight, dirt body oils that will slowly start the natural tanning process of any veg tan product. The more you use it, the more it will change!