Classical guitar harmonic notation?

For classical guitar, what I'm used to seeing is more like what Josh said above me than what kluctus said. A diamond shaped notehead for the sounding pitch might be with a little circle above or below the staff line. They usually indicate the string on which the harmonic is played (not the fret, I think...they let you figure that out) by a number enclosed in a circle.

I think a lot of this depends on the editor/publisher, though. Sometimes it's very clear, and sometimes you just get diamond noteheads with no indication of how to produce the harmonic (naturally or artificially).

Just make sure that you do it consistently throughout the piece...and the performer will greatly appreciate some sort of explanatory note at the beginning of the score.

If they are natural harmonics, write the open string and then put a diamond notehead as a dyad above where the guitarist is to touch the string. For example, the octave harmonic on the high E string would have a written E (last space, treble staff) and a diamond notehead E an octave above that (three ledger lines above the staff). The open strings sound an octave lower than notated, and so will the harmonics if you only use the node at the 12th fret.

Natural Harmonics:

use a diamond notehead for the place to finger the note. (you can optionally put a parenthetical normal notehead for the sounding pitch)

also in a circle above the note put the number of the string the note is played on.

I prefer not to see a dyads with normal notehead and diamond notehead for natural harmonics, as it's confusing as to whether it's part of the notation of the harmonic, or a chord with a harmonic and a normal note.

Artificial Harmonics:

same as in other string instruments.