Geosec thin section slides are very thin slices of rock mounted on glass microscope slides. When these are viewed in polarised light the individual crystals show bright colours, caused by optical interference. (Same effect as a film of oil on water). The colours vary with different minerals, and are used by geologists to identify the different minerals present in the rock, and hence identify the rock type. The slides are usually viewed through a specialist polarising microscope, and rotating the slide causes the colours to change. This movie was produced by a special close-up (macro) camera, and the polarising filters moved instead to produce the change of colours.