Herkimer Diamond (SiO2) is a clear quartz, however it always forms in a double termination and has a radiance approaching Diamond.
As an essence it acts as Clear Quartz, however it is morefocused on cleansing and the balancing of highly polarized energy. This essence also acts to radiate light strongly through the crystalline structures, giving intense clarity.
In all other ways it acts as Clear Quartz Essence.
(Please see Clear Quartz for more information on the action of Quartz essences in general)