Typically, an "open diamond" notehead is used to indicate a harmonic fingering. The image here is from an edition of the Cassado Cello Suite (1st mov't). It's clear from the overall context that the last note is a harmonic half-note, but visually it's the same as if it were a harmonic quarter-note. Just wondering if there's any consensus as to alternative notations that explicitly indicate the distinction between the two note-lengths?
In this case the intent is clear.
In one of the standard works on notation, "Behind Bars" by Elaine Gould, the author writes:
The diamond remains hollow, regardless of its duration. Where the rhythm is not clear from its context, differentiate minims and crochets [half and quarter notes] by writing the rhythmic values as small notes in brackets over the stave.