There is no such thing as not natural. Birds make nests. Does this mean that bird nests are unnatural? In like manner, humans make airplanes from naturally occurring elements and compounds that can be made with naturally occurring elements.
Now, if humans are a product of nature, or natural selection, then anything that humans produce is but an extension of nature. I think it is better to divide things into non-intelligently occurring and intelligently occurring objects. One comes before human intelligence, while the other comes after. Both, however, are all natural.
Take GMO, for instance. GMOs have been produced via artificial selection over thousands of years in out in the field (indeed, one look at wild versions of every vegetable and fruit in the supermarket will show that almost none of it is recognizable or desirable for food). Now that GMOs can be produced in a much shorter period of time in a lab, it is sometimes considered unnatural, but it is actually the natural future evolution of mankind.