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This is an oxymoron, is it not? A good one too!

I'm sure there are substances that can be produced by man that are identical to those found in nature. If that's what you mean.

ascorbic acid (vitamin C) can be made and is the same

water can be made combining H & O, gotta match

etc etc i'm tired

Natural and synthetic what? Sugar is natural, saccharine is synthetic. Cotton is natural, nylon is synthetic. Wood is natural but plastic is synthetic. Is that what you mean? You can easily think of more .