Tanto

This is a Japanese Tanto.  This short sword is made of 1080 carbon steel and is very sharp.

I purchased this sword to compliment my Japanese Katana which is made of 1095 carbon steel and is extremely sharp.

The higher the carbon steel number, the harder the material.  But it also makes it more brittle.  The lower the number, the softer the material, but more flexible.  If you look closely, you will see part of the "hamon" line near the base of the blade.  The reflection of light hides the rest of it along the blade.  The line is created by putting clay on part (but not all) of the blade when it is fired to harden the sword.  This makes part of the blade that does not have clay heat up more than the part that has the clay.  The sharper edge does not have the clay, so it becomes hotter.  Once plunged into water after the firing, the sword hardens and actually curves the blade as well.

The reason that you do not want the entire blade hard is for the strength of the blade when struck.  If another sword were to hit your sword, you want your sword to flex a little so that it doesn't just break.  But, you want the edge to be really sharp.  The hamon line due to the clay keeps the back part of the sword more flexible but keeps the edge part harder to allow better sharpening.

This technique has been done for centuries of sword making and is the reason why these samurai swords are so sharp!