Mineralogy is the science of minerals, the solid building blocks that make up planet Earth. From the middle of the 19th century, new optical tools for investigation and measuring were developed that could reveal the internal structure and characteristics of minerals and rocks. This led, among other things, to new knowledge about the history of the Earth and the continents.

The earliest form of mineralogical study was the description of visible crystal structure and colour. During the 18th century, crystal research entered a new era with the development of the goniometer in France. This tool allowed the angles between crystal planes to be measured with greater precision. 

The Frenchman René Haüy is often called the father of crystallography. He was the first to realise that there must be a relationship between the ”chemical structure” of a mineral and its external form. Gradually, the art developed of accurately measuring and depicting well-formed mineral crystals in order to understand their symmetry.

Revolutionary optical investigations

From the middle of the 19th century, optical methods of investigation gained prominence. The commonest technique was petrographic microscopy, the study of slices of rock that have been ground extremely thin. 

The thin sections allowed the internal structure of the rock, for example the size and shape of the constituent small mineral grains, to be studied by passing polarised light though the sample in a special microscope. This gave direct information about the composition and mode of formation of the rock – a revolutionary advance.

Instrument types that could measure the characteristics of individual crystals were also developed, such as refractometers and reflectometers which measure the refraction of light.