The Hanging Man
Tenkai Matsumoto
The thing is, I can see -why- the diamond katana guy used a rubber core.
Because he was using it as a baseball bat?
It's because he probably read about how katana use differential hardening, and the use of "skin steel" and "core steel".
Although the differential hardening is what really gives the katana a HRC 60/40, most traditional katana are constructed using softer steel for the core and harder steel for the edge. They figure this out by breaking plates of tamahagane and seeing if they shatter or tear.
I bet the guy figured he'd maximize this by taking the hardest substance he could think of for the "skin", and used the softest, most shock-absorbing substance he could think of for the "core".
Unfortunately, it works with steel because you can actually forge metals together. In order to make rubber stick to diamond you'd need like...I dunno...apoxy.
This does not a katana make.
As such, it was a slightly creative yet very bad attempt at min-maxing the benefits of a katana.