The study of mineralogy was a particular focus of the National Museum of Victoria (which eventually became Museums Victoria) in its earliest days. Over 1000 wooden crystal models were purchased between the years of 1857 and 1872 from Dr August Krantz of Bonn, Germany, one of the foremost mineral dealers of Europe at this time. Of the 1000 crystal models purchased, around 200 remain in Museums Victoria's collections today.
Crystal models are technical and educational tools used to illustrate the varieties in shape, or 'habit', particular groups of crystals can take. The range of crystal shapes possible for a material depends on the arrangement of the atoms that make it up, although it isn't always easy to see why a particular arrangement of atoms gives rise to a particular shape (for example, one of the habits taken by materials made from atoms in a cubic arrangement is a dodecahedron, or twelve-sided shape).
The model M 6831 is unusual amongst the Krantz crystal models, in that it illustrates cleavage planes, rather than a crystal habit. Cleavage is the tendency that some minerals have to break along planar weaknesses within their crystals. M 6831 is a rhombohedron, a shape into which crystals of the mineral calcite will cleave.