The ornamental design for a diamond cut necktie, as shown and described.
FIG. 1 is a front elevational view of a diamond cut necktie showing my new design, the broken line showing of the human form is for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a front perspective view of a diamond cut necktie thereof,
FIG. 3 is a front elevational view thereof; and,
FIG. 4 is a front elevational view of a diamond cut necktie in an alternate position.