The metal and the patina
Also the quality and the visual aspect of a coin's metal is a key factor in rating it.
The metal of a coin can be damaged by the time goes on.
Sometimes the defects can be caused by the low quality metal used during the minting process.
The damages could be caused by an improper cleaning or a bad restoration too.
Another aspect examined in this parameter is the patina, if present.
With time, a thin layer can appear on the surface of a coin due to exposure to light, chemical compounds, or atmospheric elements. Relevant is also the way the collectors kept it.
An old collection patina can increase the charm of a coin and its value, especially for a silver one.
The original old patinas, like the green or riverine ones on the Roman bronze coins, attract the collectors.
We remark the adjective original because in many cases the patinas are artificial, created to increase the value of a coin.
If an original patina is present on a coin, we'll report it in the data sheet resulting in a high rating in the histogram too.
Artificial patinas or modern oxidations, detected during the examination, will result in a lower rating in the histogram.
A low rating will be reserved for coins with porosity, excessive oxidations, crystallizations, et al.
On the contrary, coins with a metal without particular issues will receive a really high rating in the histogram.
Also aggressive polishing can damage the metal of a coin causing a low rating.