TomGeorge:
Hi,Looks like a Crystal to me, and when I look at my board, and the bareduino kits I have.
Tom....
Well, the schematic symbol for the CPU clock source is that of a crystal, but indeed it is a ceramic resonator. The 16U2 (USB <=> Serial) interface does use an actual crystal (it has to, else the instability and temperature drift characteristics of a resonator would make the serial interface fail).
I've heard an explanation as to why the CPU doesn't use a crystal (something to do with RFI problems) which I think is pure bunk, baloney or BS (whichever you prefer).