Where Things Come From: Rock Materials

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.

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.

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.

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.

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.

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.

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.

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.

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.

"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.