High pressure grinding roll is often referred to as HPGR, the ore is fed between two rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes micro-fracturing at the grain size level. HPGR is often considered as being more efficient than grinding mills, although they are not as common as grinding mills since they are a newer technology.