Just like the demons in Demon Slayer, Nichirin isn't a real-world metal. Instead, as described in the anime, Nichirin is made using Scarlet Crimson Ore and Scarlet Crimson Iron Sand.
Nichirin steel is a fascinating and original concept for a fantasy metal. It absorbs sunlight to obtain that light's demon-killing properties. A sword forged from this material is like a piercing ray of sunlight that a demon slayer can control.
A Nichirin blade is the steel specially made for the swords that Demon Slayer Corps members use to kill demons. According to the Demon Slayer manga (which would be the “bible” of the anime series), the blade is made out of scarlet crimson iron sand and scarlet crimson ore, which are collected from the mountains.
These blades aren't ordinary iron or steel, they're actually crafted from a fictional ore called the "Scarlet Ore" that can be found on Sunlight mountain.
Tanjiro Kamado wields a black Nichirin blade, but the symbolism of such a sword is unknown. Black blades are seen as a rarity, as demon slayers who wield them are doomed to die young. No wielder of a black sword has ever become a Hashira, either.
The Nichirin sword that Mistsuri uses is a pink whip-like sword that fits perfectly with her overall appeal and style. Her sword is said to be one of the fastest because of how incredibly quick they are to the point that not even Tengen Uzui's swords could reach speeds that fast.
Toho Co., Ltd. Black is the most mysterious color of all the blades, and is held by the main character, Tanjiro. It is a rare blade and represents the sun; it also holds a theory that the demon slayers who own this sword are not to live a long life.
After the Demon Slayer reaches the rank of Hashira, their weapons will be engraved with the words "Destroyer of Demons" ( 惡 あっ 鬼 き 滅 めっ 殺 さつ , Akki Messatsu?).
He, alongside Kyojuro Rengoku, Mitsuri Kanroji, and Gyomei Himejima, are the only four Demon Slayer heroes to be Pure Good. In addition to all three of the latter being Hashira, Tanjiro's the only Pure Good to be only be an average Demon Slayer and one of the main characters to be Pure Good.
It involves covering a Nichirin Sword with Nezuko Kamado's Exploding Blood to raise its temperature to the point it turns bright red.
1060 High Carbon Steel Blade.
Zenitsu is a master swordsman who wields a yellow Nichirin blade, a katana style blade made of scarlet ore.
Instead of a standard sword, Gyomei uses a spiked flail and axe to take out his enemies. It's a very strong and durable weapon that takes a lot of skill to use. Because Gyomei is blind, he uses the echoes of the chain to help him know the space and movements around him thanks to his acute sense of hearing.
Nichirin blades can only be forged from scarlet crimson ore and scarlet crimson iron sand, two rare minerals that can only be found in mountains with incredibly high altitudes that allow them to be consistently bathed in light year-round.
Master Swordsmith: Hotaru is a skilled swordsmith, able to forge the Nichirin Sword and repaired it multiple times each time it broke against the Lower and Upper Ranks. After Tanjiro chipped his sword in his battle against Upper Rank Six, Gyutaro and Daki, Hotaru trained more to make a stronger sword.
It is possible for a swordsman to use more than one Breathing Style technique at a time, allowing the user to take advantage of the advantageous aspects of both techniques. Tanjiro Kamado, for example, can use both the Water Breathing and Sun Breathing styles simultaneously.
After returning to his human form, Tanjiro loses the ability to see from his right eye, the iris of which is now a dull pink-gray color and the pupil black rather than its original white.
Tanjiro makes use of Water Breathing for most of the series. Taught to him by Sakonji during his training in the mountains, it is one of the five main styles derived from Sun Breathing.
His blades are engraved with the phrase Destroyer of Demons ( 惡 あっ 鬼 き 滅 めっ 殺 さつ , Akki Messatsu?).
When Tanjiro first got his Nichirin Sword on Demon Slayer, and it changed color to match the personality of its user like all Nichirin weapons do, many fans were surprised that Tanjiro's katana turned black.
To become a member of the Demon Slayer Corps, he had to clear the Final Selection. During this event, Tanjiro received a massive blow to his head from the Hand Demon that killed many of Urokodaki's students. This increased the size of his scar and changed the color as well.
Nichirin Blades are forged with Scarlet Crimson Iron Sand and Scarlet Crimson Ore, both of which are found on high mountains like the Sunlight Mountain, which is perpetually bathed in sunlight all throughout the year.
It was Yoriichi's sword, he found it inside Yoriichi Type Zero doll when he destroy the head. And then hotaru took the sword to fix it. Tanjiro also use Kyojuro's tsuba in this sword.
Standard Nichirin Katana: As a Demon Slayer, Yoriichi carries around a standard sized and shaped Nichirin katana that is deep black in color, but changes bright red during combat.