Metals are inspired by the Eagle flying towards the sun.
They also symbolise the “Yellow Eagle” which is the Indian name for Goro Takahasi.
Metals are crafted based on the Yin and Yang.
Silver means Yin (陰) and gold means Yang (陽).
This philosophy was taught by a monk who Goro respected.
As you can see, the Metals are one of the signature goro's items similarly to the Feathers and Eagles.
It is also an essential item in building your setup.