It depends on what your are starting from. If it's a sine wave, yes
even harmonics can be made from diode non-linearities.

The OP has a inverter-style XTAL oscillator, output very nearly
square.

A square wave is rich in odd harmonics, a perfect square wave has NO
even harmonics.



Oh! I see what you're talking about...
I presumed that he was starting with a single spectral component (sine wave) and wanted to end up with another single spectral component.


Joe
W3JDR