Flat faces at the top of a crystal are called windows. Windows may form in a diamond shape, some large, some small, which facilitate clarity of mind and the organization of information received from different levels of being. Gazing into a diamond window takes you deep inside yourself or enables you to read information for someone else.
A true diamond window is large and connected to the apex and the base, but even small diamond windows can assist the balance between the spiritual and the material worlds, facilitating living in everyday reality while at the same time being connected to a greater reality. Diamond windows provide a doorway into other levels of being and causes of dis-ease.
Source: The Crystal Bible by Judy Hall