Where Things Come From: Rock Materials

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.

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.

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.

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.

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.

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.

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.

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.