“It is said the warrior’s is the twofold Way of pen and sword, and he should have a taste for both Ways.”
— Miyamoto Musashi
Both a blade and a pencil have hard materials at their core (a katana can have many layers of different hardness, but the general principle is to have a hard metal surrounded by a softer one). For the pencil, it’s the graphite, and for the katana, it’s the steel with high carbon content. For both instruments, the hard core forms the working part, which can be sharpened to a fine point/edge.
The hard core is enclosed in a relatively soft material – wood in the case of the pencil and low-carbon steel in the case of the sword. Without the outer set shell, neither instrument would be practical to use, because the core it too brittle to withstand the pressure of the artist’s hand or a strike of an enemy’s sword. Likewise, a soft, mono-layered instrument without a core would be a compromise at best in terms of cutting/drawing quality. Think about a bronze sword or a crayon – neither is particularly strong, and neither can be sharpened to a fine point or edge.
A pencil that we use today is a European invention. Hand-carved wooden holders with graphite core were first made in England in 1564, and a Czech company Kohinoor patented and mass-produced pencils that were very similar to modern ones in the 19th century.
Europeans also made multi-layered blades, but the technique was refined and taken to the level of an art in Japan in middle ages.
I find it curios how these tools from two unrelated fields of application (cutting and writing) evolved along similar design paths, because in both fields similar qualities are valued – sturdiness and ability to be sharpened.