system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] What do Witcher medallions do and how do they work?

We know that they are meant to warn the wearer of danger and magic by agitating. But how? I can understand it responding to a magical aura by wobbling or something because that is simple. But at a few points Geralt's amulet manages to warn him or more physical dangers, like being ambushed, but how in the hell does that work?

We also know that they are made from silver and infused with elemental energy at what can only be assumed to be a layline, but also that a replica can be made from crystal by a sorceress with presumably limited experience.
gpt
>The witcher medallion is not only an insignia of our profession, it is also one of its tools. It has numerous useful capabilities that are accessible, of course, only to one who possesses the necessary knowledge and training. First of all, it reacts to the presence of sorcerous auras in the immediate surroundings, making us aware of nearby spellcasting, active illusions, or magical creatures. It also warns the owner of sudden dangers, thus providing an additional moment to react. Keep in mind, though, that the medallion is not infallible. A very strong magical aura, such as those found in Places of Power or during the solstices, can provoke aberrant reactions. In large cities, where people widely use simple magical amulets or sorcerous alarm systems to protect their homes, coin, or ruttish wives, the medallion can also behave abnormally.
>

As to how it works, the explanation can't really become more detailed than "magic". It is a magically active item, and magically programmed to give reaction to the approach of appropriate things.