Crushed stone or angular rock is a form of construction aggregate, typically produced by mining a suitable rock deposit and breaking the removed rock down to the desired size using crushers. It is distinct from gravel which is produced by natural processes of weathering and erosion, and typically has a more rounded shape.
Crushed stone is one of the most accessible natural resources, and is a major basic raw material used by construction, agriculture, and other industries. Despite the low value of its basic products, the crushed stone industry is a major contributor to and an indicator of the economic well-being of a nation. The demand for crushed stone is determined mostly by the level of construction activity, and, therefore, the demand for construction materials.
Crushed stone is a high-volume, low-value commodity. The industry is highly competitive and is characterized by many operations serving local or regional markets. Production costs are determined mainly by the cost of labor, equipment, energy, and water, in addition to the costs of compliance with environmental and safety regulations. These costs vary depending on geographic location, the nature of the deposit, and the number and type of products produced. Crushed stone has one of the lowest average by weight values of all mineral commodities. The average unit price increased from US$1.58 per metric ton, f.o.b. plant, in 1970 to US$4.39 in 1990. However, the unit price in constant 1982 dollars fluctuated between US$3.48 and US$3.91 per metric ton for the same period. Increased productivity achieved through increased use of automation and more efficient equipment was mainly responsible for maintaining the prices at this level.
According to the United States Geological Survey, 1.72 billion tonnes of crushed stone worth $13.8 billion was sold or used in 2006, of which 1.44 billion tonnes was used as construction aggregate, 74.9 million tonnes used for cement manufacture, and 18.1 million tonnes used to make lime. Crushed marble sold or used totaled 11.8 million tonnes, the majority of which was ground very fine and used as calcium carbonate.
In 2006, 9.40 million tonnes of crushed stone (almost all limestone or dolomite was used for soil treatment, primarily to reduce soil acidity. Soils tend to become acidic from heavy use of nitrogen-containing fertilizers, unless a soil conditioner is used. Using aglime or agricultural lime, a finely-ground limestone or dolomite, to change the soil from acidic to nearly neutral particularly benefits crops by maximizing availability of plant nutrients, and also by reducing aluminum or manganese toxicity, promoting soil microbe activity, and improving the soil structure.
In 2006, 5.29 million tonnes of crushed stone (mostly limestone or dolomite) was used as a flux in blast furnaces and in certain steel furnaces to react with gangue minerals (i.e. silica and silicate impurities) to produce liquid slag that floats and can be poured off from the much denser molten metal (i.e., iron). The slag cools to become a stone-like material that is commonly crushed and recycled as construction aggregate.
Crushed stone or 'road metal' is used in landscape design and gardening for gardens, parks, and municipal and private projects as a mulch, walkway, path, and driveway pavement, and cell infill for modular permeable paving units. As a mineral mulch its benefits include erosion control, water conservation, weed suppression, and aesthetic qualities. It is often seen used in rock gardens and cactus gardens.