How to distinguish the authenticity of hamon
The hamon is a tempering line and visual point of demarcation on a Japanese sword which separates the harder edge from the softer spine and is a result of a process known as differential hardening. This is the process where the blade is quenched so that the edge is harder than the spine or body of the blade. The spine of the blade is coated with a clay mixture, then heated and quenched. The thick clay coating on the spine acts like an insulator and causes the coated portion of the blade to cool more slowly.
The appearance of a real hamon is like an acid etched hamon: it’s white and cloudy. But a real hamon seems to “glow” under light and the blade needs to be at a certain angle to view it. Up close, you’ll see tiny dots/specks along and between the harder martensitic steel at the blade’s edge and the rest of the blade which is the softer pearlitic steel at the center and back of the sword.
Hamon can be wavy or straight.
Acid etched hamon can actually fool a beginner because it looks like a real hamon: it’s white, cloudy, and there are no scratches from wire brushing, but it is not natural, nor authentic. This is made using various chemicals to etch the hamon. After which the blade is polished with a fabric buff to make the etched hamon look smooth on the surface of the steel. This looks better than a wire brushed hamon, but this fades when you clean the blade with metal polish. This hamon is visible at any angle you look at it and does not need special lighting.
A real hamon has dimensionality, softness, organic qualities, granularity, a certain color difference (often a warm tone), a certain sparkle, typical variations of density/brightness, a habuchi (dividing line) of a certain form, and many hataraki (visible metallurgical activities) etc. Oil quench hamon tend to be relatively featureless (no/little hataraki), a little bluer, a narrower habuchi, dark shadows under the peak. A katana with a natural hamon is more expensive.
Learning to differentiate between authentic hamon and a fake one is crucial when acquiring a clay-tempered sword.
In fact, it is the only way to be sure at first glance that the sword you are buying is of good quality, and not a simple imitation.