Reply by Prof Peter Clift Sand is always a sedimentary rock, but of course it is made up of grains eroded from igneous and metamorphic sources. Although an SEM can be useful, it is rare that the texture of the sand alone will tell you where it is from unless they grains are very rounded, which is typical of desert dune sands. The method you choose depends on what the possible sources are, how much sample you have, and how much money you are willing to pay. Generally quartz dominated many sands but is often not very characteristic. A better approach might be to look at the grains which are not quartz and see what types you have and what their relative abundance is like. If this looks promising then electron probe analysis of these grains or even dating of grains like zircon can be very effective
Finally, the identification of minerals by petrological microscope is not essential in a provenance study but it is common because it is relatively cheap, easy and quick.