Chemical reaction: When silver is exposed to the air for a long time, it will oxidize and form silver oxide on the surface, which is brown or gray-black. When it encounters sulfur in the air, it also produces black silver sulfide.

Personal physique: Some people's sweat contains more acid and sulfur content. Therefore, different people will have different results when using it. Some people will turn black after wearing it for a few days, while others will become brighter the more they wear it.