Where Things Come From: Rock Materials

Most of us buy rock materials—stone, gravel, clay and other basic natural substances—at a store. Stores get them from warehouses, which get them from processors or shippers. But they all begin somewhere in nature, where a raw ingredient that cannot be manufactured is taken from the ground and brought to the market without being transformed by processing. Here's where rock materials come from.

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Boulders

Boulders and talus
Boulders and talus in Oregon. Boulders and talus in Oregon; Geology Guide photo
Landscapers can procure just the right boulder for a yard or atrium from a variety of sources. Smooth "river rock" is extracted from sand-and-gravel deposits. Rough "natural rock" is mined from quarries using explosives and heavy machinery. And weathered, mossy or lichen-covered "surface rock" or fieldstone is harvested from a field or a talus pile.

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Building Stone

Stone wall
Stone wall built of irregular blocks. Stone wall built of irregular blocks; Geology Guide photo
Any rock suitable for construction can be called building stone, but it usually signifies unsurfaced blocks that are assembled into walls by masons. It ranges from material of random size and shape to cut blocks (ashlars) with unfinished surfaces, or veneers of the same type. This material generally comes from quarries to ensure a consistent look, but gravel deposits can also produce it.

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Clay

Former clay mine
Former clay mine in Golden, Colorado. Former clay mine in Golden, Colorado; Geology Guide photo
Clay is mined from beds of clay or made by grinding shale. It is mined mostly from surface pits, although there are some subsurface workings. Clay companies take great care in choosing their sources because clay is used for many different purposes. The raw material is dried, pulverized, screened, blended and wetted again before shipping. Most clay is processed for industrial use (to make bricks, tiles etc.), but pottery clay and pet litter are close to their natural state.

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Coal

Coal
Bituminous coal. Bituminous coal; Geology Guide photo
Coal does not occur everywhere, but only in sedimentary rocks of certain ages. Coal is produced from large surface pits and underground mines, depending on the grade and bedding of the material. It is washed, crushed and screened into different sizes suited for power production, smelting or other purposes. The industrial coal market is worldwide; the market for home heating with coal is local.

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Cobbles

Cobbles
Cobbles set next to a city sidewalk. Cobbles set by a city sidewalk; Geology Guide photo

Cobbles, used for paving and walls, range from fist to head size (geologists use a different size range, 64 to 256 millimeters). Smooth cobbles come from riverbeds or beach deposits. Rough cobbles are produced in quarries by crushing or chopping and dressed by tumbling rather than by hand-finishing.

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Crushed Stone

Road metal
Crushed stone in a railroad bed. Crushed stone in a gravel quarry; Geology Guide photo

Crushed stone is manufactured aggregate, an essential material for building roads (mixed with asphalt), constructing foundations and railbeds (road metal) and making concrete (mixed with cement). For these purposes it can be any type of rock that is chemically inert. Crushed limestone is widely used in the chemical and energy industries. Crushed stone can be produced from bedrock in stone quarries or from river deposits in gravel pits. In either case, it usually comes from a nearby source and is the most common purpose for opening a quarry. The crushed stone (often labeled "gravel") for sale in your garden-supply store is selected for its color and strength, and it may come from farther away than the stuff used in roadbeds.

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Dimension Stone

Stone fountain
Haupt Fountain in Washington DC is a single slab of dimension stone. Haupt Fountain in Washington DC; Geology Guide photo

Dimension stone refers to any stone product that is produced in slabs from quarries. Stone quarries are pits where large blocks are cut using abrasives and saws or split using drills and wedges. Dimension stone refers to four main products: ashlars (rough-surfaced blocks) used to build walls using mortar, facing stone that is trimmed and polished for decorative use, flagstone, and monumental stone. All of the variety of rock types that geologists know fit just a handful of commercial rock names: granite, basalt, sandstone, slate, limestone and marble.

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Facing Stone

Facing stone
Verd antique facing stone. Verd antique facing stone; Geology Guide photo
Facing stone is a category of dimension stone that is precisely cut and polished to add beauty as well as durability to buildings both outside and inside. Because of its high value, facing stone is a worldwide market, and there are hundreds of different varieties to use in cladding for outside walls, inside walls, and floors.

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Flagstone

Phyllite flagstone
Phyllite flagstone. Phyllite flagstone; Geology Guide photo

Flagstone is sandstone, slate or phyllite that is split along its natural bedding planes and used for floors, pavement and paths. Smaller pieces of flagstone may be called patio stone. Flagstone has a rustic and natural look, but it comes from large, modern quarries.

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Granite Countertops

Countertop granite
Commercial granite. Polished granite; Geology Guide photo

"Granite" is a term of art in the stone business; a geologist would give a lot of commercial granite another name, such as gneiss or pegmatite or gabbro ("black granite") or even quartzite. And marble, a softer rock, is also used for countertops that get less wear. Be that as it may, granite countertops and other stone pieces in the home begin as quarried slabs from all over the world. Slabs are custom-cut in a local shop for the best fit, although simpler pieces like a vanity top may come readymade.

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Gravel

Gravel
Gravel. Ferruginous gravel; Courtesy Robert Van de Graaff

Gravel is natural rounded sediment particles larger than sand (2 millimeters) and smaller than cobbles (64 mm). Its overwhelming use is as aggregate for concrete, roads and construction projects of all kinds. Every state in the union produces gravel, which means that the gravel you see in your neighborhood comes from nearby. It is produced from current and former beaches, river beds and lake bottoms, and other places where coarse sediment has been laid down for a long time. Gravel is dug up or dredged, washed and screened before being taken to market, usually by truck. Landscaping gravel is a more select product, chosen for its color and consistency. In areas without enough gravel, crushed stone is the usual substitute and may also be called gravel.

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Gravestones (Monumental Stone)

Monumental stone
Graveyard statue. Marble angel, granite grave; Geology Guide photo
Grave markers are part of the monumental stone segment of the dimension stone industry. Monumental stone also includes statues, columns, benches, caskets, fountains, steps, tubs and so on. Raw stone is quarried and then carved by skilled artisans following standard patterns and models before shipping. Locally, before the stone is installed, another set of artisans does any final customization, such as carving names, dates and ornaments. Sculptors are also a small but prestigious part of this market.

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Greensand

Glauconite greensand
Glauconite. Glauconite; courtesy Ron Schott (Flickr CC BY-NC-SA 2.0)
Greensand is a sediment containing the mineral glauconite, a soft green silicate of the mica group that acts as a gentle, slow-release potassium fertilizer and soil conditioner for boutique gardeners (industrial farmers use mined potash). Greensand is also good for filtering iron from water supplies. It is mined from sedimentary rocks (glauconitic sandstone) that originated on the shallow seafloor.

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Lava Rock

Scoria
Scoria or lava rock. Scoria; Geology Guide photo

Geologically, the landscaping product known as "lava rock" is pumice or scoria—lava so charged with gas that it hardens to a frothy texture. It is mined from young volcanic cones and crushed to size. Its light weight helps lower the cost of shipping. The great majority of this material disappears into concrete building blocks. Another use is in the fabric treatment known as stonewashing.

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Sand

Sand
Black sand. Black sand of Hawaii; Geology Guide photo
Sand is sediment between 1/16th and 2 millimeters in size. Ordinary sand is abundant and widespread, and chances are what you buy in the nursery or the hardware store comes from a sand-and-gravel pit or quarry nearby. Sand is mostly from river beds rather than the seashore, because beach sand has salt in it that interferes with concrete setting and garden health. High-purity sand is classified as industrial sand and is somewhat scarcer. At the quarry, raw sand is washed, sorted and blended to make various products suited for concrete, soil amendment, base material for hardscapes, pathways and so on.

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Soapstone

Soapstone outcrop
Soapstone Ridge, Georgia. Soapstone outcrop, Georgia; courtesy Jason Reidy (Flickr CC BY 2.0)

Manufacturers argue that soapstone is superior to granite for kitchen counters; it is also used for laboratory bench tops and other specialized purposes. Soapstone has a rather limited occurrence because it usually arises from peridotite, another limited rock type, by metamorphosis. Small deposits have been mined since ancient times because the stone is so easily carved, but today's soapstone is shipped around the world from a few large workings.

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Suiseki Stones

Suiseki
Suiseki "mountain stone". Suiseki "mountain stone"; Geology Guide photo

Suiseki, the art of selecting and presenting natural stones as cabinet pieces, arose in Japan but is widely practiced by lovers of stone shapes and textures. China and neighboring countries have similar traditions. You might consider suiseki the ultimate refinement in ornamental boulders. The most interesting stones are found in the headwaters of rivers and places where weathering has sculpted exposed bedrock without wearing it down into rounded shapes. Like other fine art, suiseki stones are acquired from the individuals who collect and prepare them, or from specialty shops.

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Track Cinder

Cinder track
Cinder track. Cinder track: altrendo / Getty Images

The lightweight grit used on running and riding tracks is a finely ground pumice or "lava rock." Cinder is another name for volcanic ash and lapilli.