Kinsuji (金筋) is a term used in the context of Japanese swords. It refers to a thick, shining line that appears along the grain of the blade. This feature is created by the presence of a different iron structure called ""Martensite"" within the surrounding iron structure called ""Troostite"".
In more detail, ""Martensite"" and ""Troostite"" are terms used to describe different structures of iron that result from the heat treatment process in sword making. ""Martensite"" is a very hard and brittle structure that forms when steel is cooled rapidly, while ""Troostite"" is a softer and more ductile structure that forms when steel is cooled more slowly.
The term ""Nie"" (沸) is used to describe the visible particles of hardened steel that appear along the cutting edge of the blade. When these particles form a thick line along the grain of the blade, this feature is referred to as ""Kinsuji"".
Therefore, ""Kinsuji"" adds a unique and beautiful visual effect to the blade, enhancing its overall aesthetics and value."