To make a slide of a rock involves cutting it to a very exact thickness. This is because, under the petrological microscope which geologists use, every mineral cut to the correct thickness has a characteristic structure and colours - the petrological microscope is described on Rob's website, here. Not only does the microscope enable every mineral to be identified, but it allows even more to be learned by rotating the slide, when the colours in the minerals vary, also in a characteristic way. Thus much can be learnt of the history of the formation of the rock, and of its chemical composition.
Rotating these slides under the microscope produces dazzling colour effects. While there is no substitute for seeing this show 'for real', Rob has created videos to illustrate the colour changes - something which is a tribute as much to his technical expertise with a camera and computer as to his knowledge of the microscope. Here's one video, which shows eucrite, the main rock type of Ardnamurchan's volcanic ring complex -
....while here's another, this time of a schist from the USA -
Both these videos are on Rob's website, here, where he describes how each of the minerals can be identified - but he does warn that he's still working on them!
Many thanks to Rob Gill at GEOSEC Slides