I don't know if they are tougher but it might not be true. I know a lot of brown eyed and brown haired people but not all of them are tough.

it is light brown in colour but it is hard and tough. It is heavy and durable outside. Gets harder with age

A cow is is normally used in the manufacturing of a brown leather satchel. The skin is tough and durable but it can have a smooth or rough surface. Brown or black are popular for this type of leather.

Sounds like your tree is diseased.