Why did they try gold before for a tooth?

Why try a gold coin on a tooth? Now it will probably never occur to anyone to check the authenticity of gold in such a strange way. But frequent episodes in films about pirates where they try to bite a coin for such a purpose is not an invention of a director or screenwriter. What is the main secret of this method? To make sure of the strength and hardness of gold, it is enough to conduct a simple experiment available to everyone - to try to bend any gold jewelry. It is unlikely that this undertaking is successful. It can be assumed that before the coins were tried for a tooth for this purpose - real gold should have been maintained. But there is a small nuance: pure gold (which, by the way, is not 100%perfectly clean) - one of the softest metals. Therefore, it can be deformed without making special efforts. It turns out when we see how the hero of the cartoon or movie tries a coin on the tooth, then he expects to see a dent on him as a sign of authenticity? This could work with pure metal, but even during the reign of the Tudors in the United Kingdom and in America, coins were minted from a alloy of gold and copper. So the money became much harder and more durable.