According to the photos below, this is the Xperia Z3's metal frame. It is treated with a PVD coating, which will provide a more corrosion-resistant interface, better impact resistance and resilience to scratches. It is also said to be more environmentally friendly.
The source of the photos claimed that the Xperia Z3 will ignore the QHD-trend, instead opting for a 1080p display, a beefy Snapdragon 805 CPU clocked at 2.7GHz, 3GB of RAM, and a 20.7MP main camera. This doesn't sound like too much of an upgrade over the Z2, but who knows, maybe Sony is working on something special.
Xperia Z3 metal frame?
via G For Games