Long-standing wish. There is a triangle and others under Special Windows, but no diamond.
Try the download from David posted here:
http://archicad-talk.graphisoft.com/viewtopic.php?p=43166#43166
Otherwise, you'll have to make it yourself, which isn't too hard. If it is merely an opening with no frame or casing, use the "W Rhombus Opening 12" object with the Window tool and put a thin wall in the opening for glass.
If you need frame/casing, there are various tutorials here and in the help file for creating custom windows without programming. If needed, someone will point you to them.
😉
Karl
