After trying many think to make a leather marble, finally, I am able to create a nice looking marbled leather. What I like about this process, is I can play with colors. It is a fun process when you get it right and surely it give your leather pieces a unique look.
Here is an example of a notebook cover or passport cover.
https://www.4donis.com/product-page/one-of-a-kind-leather-marble-field-note-cover-with-pen-holder
Here is another example of a marble leather wallet
https://www.4donis.com/product-page/only-one-item-leather-marble-wallet-business-card-holder-credit-card-holder
https://www.4donis.com/product-page/elegant-handmade-leather-card-holder-credit-card-holder-minimalist-wallet
Peace