There are few minerals which can be attributed to advancements in multiple fields of science; calcite is one of them.
It displays bi-refringence, an optical property where light is split into two rays at right angles to each other. Bi-refringence is due to the underlying lattice structure of a mineral All minerals, except those with completely symmetrical lattices, are bi-refringent, but the effect is generally not obvious in hand sample. However the effect is very obvious in calcite and people have been utilising the optical properties of calcite for hundreds of years. The vikings are said to have used it for navigation. It was also used a polar filter (with some fiddling) in optical equipment until the manufacture of polarising coatings and films made it obsolete.
Calcite also has a very distinct cleavage, when broken repeatedly it eventually always makes paralloid fragments and these fragments will not reduce into other shapes. This observation was used by early crystallographers, who realised that these structures represented basic units within the crystals themselves.
