There is no un-natural coffee. I am sure there will be some day, depending on what the decidedly watery term "natural" means to you. But in this context natural coffees are the dry-processes kind, also called unwashed coffees.
In this method the coffee fruit is picked from the tree and laid out to dry in the sun until it becomes a hard, dark brown pod. In most cases the pod dries slower than coffee that had had the skin removed and the fruit mucilage fermented or scrubbed off, wet-processed or washed coffees. The slower drying in the presence of the fruit and skin changes the flavor of the roasted natural coffee. There are also theories about other chemical differences in the seed of a natural coffee since the biological trigger telling the seed that it can start to germinate is not set off in a natural coffee. But that's not something I can go into since I am posting this from a phone!
The cherries picked for natural lots are no different from washed lots. But in the wet mill there are methods to remove unripe or defective fruits that do no exist in the simple "lay it in the sun to dry" method. That means an extraordinary amount of hand labor is needed to remove bad cherries from the drying beds.
You would think that making sure only ripe cherries were selected for drying, to avoid all the subsequent labor to pick them out later. But in Ethiopia and many other places this is not the case. A huge spectrum if unripe to ripe fruits are included, largely due to local culture and business practice. If you turn away farmers who bring in a mix of cherries then they won't return. They will go to the mill down the road. Even paying more wont ensure getting better quality fruit. Small holder farmers in Ethiopia are set in their ways. The only way to control fruit picking is to own the farm, and their are few large estates in the best coffee areas of Ethiopia.
I include above a picture of cherry sorting just beginning on a set of raised beds at the privately owned Aricha mill in Yirga Cheffe from my visit last week. When this was owned by Abdullah Bagersh the top coffees here were called Idido Misty Valley. They maintain some of the Bagersh practices, in the most part because they are the same local workers who learned by his standards. -Tom