What is an aggregate in mineralogy ?

Crystalline aggregate

This term designates an assembly formed of numerous crystals (automorphic to xenomorphic) of the same or different nature, thus constituting a rock. Lapis lazuli (right photo) is an aggregate of lazurite, calcite and pyrite. If the crystals are all of the same mineral species, then we speak of monomineral rock (such as quartzite, which consists only of quartz).

Colloidal aggregate

This term designates a loose and disordered agglomeration of small particles of matter : colloids. The finest example of a colloidal aggregate in the mineral kingdom is the opal.