Herkimer Diamonds have a Trigonal crystal system and are in the mineral class of oxides in the Quartz group. They are a unique form of Quartz that generally crystallize as double-terminated, very clear crystals. Their compact formation makes them slightly harder than regular quartz, but not as hard as diamonds.
This pseudo diamond received its name from its place of discovery, Herkimer, New York. They are also found in some areas of Spain and Mexico. The ones I currently have come from Pakistan.