Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz.
As the term is used by geologists, sand particles range in diameter from 0.0625mm (or 1⁄16 mm, or 62.5 micrometers) to 2 millimeters. An individual particle in this range size is termed a sand grain. The next larger size class above sand is gravel, with particles ranging from 2 mm up to 64 mm (see particle size for standards in use). The next smaller size class in geology is silt: particles smaller than 0.0625 mm down to 0.004 mm in diameter. The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour).
ISO 14688 grades sands as fine, medium and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In the United States, sand is commonly divided into five sub-categories based on size: very fine sand (1⁄16 - ⅛ mm diameter), fine sand (⅛ mm - ¼ mm), medium sand (¼ mm - ½ mm), coarse sand (½ mm - 1 mm), and very coarse sand (1 mm - 2 mm). These sizes are based on the Krumbein phi scale, where size in Φ = -log base 2 of size in mm. On this scale, for sand the value of Φ varies from -1 to +4, with the divisions between sub-categories at whole numbers.

Close-up of sand from a beach in Vancouver, showing a surface area of (approximately) between 1-2 square centimetres.

Heavy minerals (dark) in a quartz beach sand (Chennai, India).

Gravel is a rock that is of a specific particle size range. Specifically, it is any loose rock that is larger than 2 mm (0.079 in) in its smallest dimension (about 1/12 of an inch) and no more than 64 mm (2.5 in). The next smaller size class in geology is sand, which is >0.0625 to 2 mm (0.0025 to 0.0787 in) in size. The next larger size is cobble, which is >64 to 256 mm (2.5 to 10.1 in). Gravel can be sub-categorized into granule (>2 to 4 mm/0.079 to 0.16 in) and pebble (>4 to 64 mm/0.16 to 2.5 in). One cubic yard of gravel typically weighs about 3000 pounds (or a cubic meter is about 1,800 kilograms).
Gravel is an important commercial product, with a number of applications. Many roadways are surfaced with gravel, especially in rural areas where there is little traffic. Globally, far more roads are surfaced with gravel than with concrete or tarmac; Russia alone has over 400,000 km (250,000 mi) of gravel-surfaced roads. Both sand and small gravel are also important for the manufacture of concrete.

Gravel (largest fragment in this photo is about 4 cm).

A gravel road in Terre Haute, Indiana.

Gravel being unloaded from a barge

Clay is a naturally occurring material composed primarily of fine-grained minerals. Clay deposits are mostly composed of clay minerals, a subtype of phyllosilicate minerals, which impart plasticity and harden when fired or dried; they also may contain variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits

The Gay Head cliffs in Marthas Vineyard consist almost entirely of clay.

Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and other diluted solvents. These solvents, usually acidic, migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary deposition process after they have been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy depositional environments such as large lakes and marine deposits.
Primary clays, also known as kaolins, are located at the site of formation. Secondary clay deposits have been moved by erosion and water from their primary loca

Deforestation for clay extraction in Rio de Janeiro city - Brazil. The picture is of Morro da Covanca - Jacarepaguá.

From Wikipedia, the free encyclopedia : Extraction of sand, gravel and clay