unnatürlich
unnatural
Not corresponding to nature; artificial, affected, or in a way that is perceived as strange or not normal.
Die Farbe ihrer Haare wirkte völlig unnatürlich.
The color of her hair looked completely unnatural.

unnatural
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unnatural
Not corresponding to nature; artificial, affected, or in a way that is perceived as strange or not normal.
Die Farbe ihrer Haare wirkte völlig unnatürlich.
The color of her hair looked completely unnatural.

The word consists of the prefix 'un-', which expresses the opposite, and the adjective 'natürlich' (natural). Something that is 'unnatürlich' (unnatural) is therefore 'not natural'.
Die unnatürliche Stille im Wald machte ihm Angst.
The unnatural silence in the forest frightened him.
You can often recognize the meaning from context: If 'unnatürlich' (unnatural) refers to things, colors, materials, or movements, it usually means 'artificial'. If it refers to human behavior such as smiling or speaking, it means more like 'affected' or 'inauthentic'.
Der Joghurt hatte einen unnatürlich süßen Geschmack.
The yogurt had an unnaturally sweet taste.
When referring to affected behavior, 'unnatürlich' (unnatural) is often combined with verbs of expression. Typical examples are 'unnatürlich lächeln' (to smile unnaturally), 'unnatürlich klingen' (to sound unnatural), or 'sich unnatürlich verhalten' (to behave unnaturally).
Als er lügen musste, klang seine Stimme plötzlich unnatürlich hoch.
When he had to lie, his voice suddenly sounded unnaturally high.
natural / of course
'natürlich' and 'unnatürlich' are direct opposites. 'natürlich' fits when something comes from nature or seems genuine and unaffected. 'unnatürlich' is used when something seems artificial, strange, or not genuine. With people, it is often about behavior: a natural person seems relaxed and authentic, while an unnatural person seems affected. The words can be confused because both talk about nature or genuine behavior, but the evaluation is exactly the opposite.

Ihr Lächeln wirkte unnatürlich, weil sie eigentlich nervös war.
Her smile seemed unnatural because she was actually nervous.
Vor der Kamera bleibt mein Bruder ganz natürlich und spricht wie immer.
In front of the camera, my brother stays completely natural and speaks the way he always does.