Exotic grains of sand, magnified 250 times

The primary component of typical beach sand is quartz, or silica (SiO2). Quartz is a hard mineral which, not having any cleavage planes, does not fracture easily. 

Quartz is found in many types of rocks. Granites, shown below, are common plutonic rocks that have a high quartz content. 

The quartz minerals are, in the rocks shown at left and below, clear to white. They can display any color, however, from the black of flint to the irridescence of opal. 

The rocks in the background of the image below are polished, showing the minerals of the rock in cross section, whereas the outlined, inset box shows rocks as they were found, both rounded and angular.

All rocks are subject to weathering - besides plutonic rocks there are also sedimentary, volcanic and metamorphic (altered by heat and/or pressure) rocks. 

Some sedimentary rocks in particular, such as sandstone, shown below, have a large amount of quartz sand alreadu incorporated into them.

They act as a ready source of sand, easily eroded quartz grains that have already been separated from their original mineral matrix. 

Grains from these rocks may be rough or already smooth, depending on the conditions in which they were originally formed.

A rock is weathered mechanically by rain, heat fluxes, expansion of ice and roots, and impacts of rock upon a surface after a piece has been detached from the bedrock. The rounded clast seen within the blue inset box of the image above has been worn by stream-action. 

The sharp pieces which break off join the sand in the stream system. The non-quartz components are more likely to fracture, creating zones of weakness which are preferrentially eroded as the clast moves downstream, occassionally knocking a quartz grain loose.

Chemical weathering focuses on the alteration of a substance by chemical reactions. Rains bearing acid, groundwater, soil processes, and biotic (organism-caused) effects all act to affect the rock. 

Some non-quartz minerals break down into clay minerals while quartz, and other chemically-resistant materials are isolated and washed downstream to the ocean. Chemical and physical weathering may both erode a rock at the same time.

Depending upon how far the sand travels before it gets to the ocean it may be either angular or very rounded and smooth. The more distance a grain travels the more likely it is to be knocked into other clasts, breaking off sharp edges, leaving a smoother, more rounded surface. 

The sand grains shown in the scanning electron microscope image at right have a high level of rounding, lacking angular edges. The filaments are small roots from nearby plants.

https://geomaps.wr.usgs.gov/parks/coast/sand/sand.html

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