Only the Treasure Finder and Fortunate Cat are non-skill equipment that can be fused with precious metals, both do not start as silvered and need Silver Ore to begin to be upgraded. All equipment that adds to any attribute can be fused with other precious metals. Equipment that grants a skill often can be fused with precious metals, with these exceptions:
Any lens equipment, wristband of focus, feather of flee, freedom brooch, and traveler's talisman.