What is gravel?
Gravel is similar to crushed stone because it is a type of rock, but these two materials don't occur in the same way. While crushed stone is a product of human manufacturing, gravel occurs through natural processes.
A geological definition of gravel is “a natural material that consists of water-transported materials and usually has a rounded shape as a result of the water transport.”
One of the key differences in the look and feel of crushed stone and gravel is the edges of the stone. Crushed stone often has an angular and jagged edge because of the crushing process. Gravel, on the other hand, typically has a very smooth texture and surface because of the natural weathering and wear of being exposed to running water.
Unlike crushed stone, gravel is usually sold and used in its natural state. If it is crushed, gravel loses its unique smooth rounded texture and simply becomes crushed stone.
It is important to note that some contractors refer to any stone within certain size ranges as “gravel,” even if it is crushed stone.
For example, depending on the region of the United States, crushed stone in the size of a few mm to 2 inches is referred to as “gravel,” even if it has been crushed and is no longer smooth.
There are three types of natural rock and stone: igneous, sedimentary, and metamorphic.
Igneous rock: Basalt, Gabbro, Granite, Obsidian, and Pumice.
Sedimentary rock: Breccia, Conglomerate, Limestone, Sandstone, and Shale.
Metamorphic rock: Gneiss, Marble, Metaquartzite, Schist, and Slate.
Igneous: Igneous rocks are formed after molten rock or lava solidifies.
Sedimentary: Sedimentary rocks are formed over time by the accumulation of small particles cementing together, often producing layered rock.
Metamorphic: Metamorphic rocks become changed through intense heat or pressure. Similar to clay hardening in an oven, metamorphic rocks become very hard and crystallized by intense heat or pressure.
Common Types of Gravel
If you go to a creek or river, you'll see all types of rocks, both large and small. These larger rocks can be used for foundations or other building projects, but typically gravel is screened, and only the smaller pieces are used.
There are other types of gravel, but pea gravel and crushed stone - rock are among the most common. You can also purchase gravel based on color, but this tends to be more expensive since it takes more work to screen for color than size.
So what are some of the most common rocks used in construction, and what are they used for?
Basalt: An igneous rock often used for road pavement or concrete aggregates. It's also used for masonry projects.
Granite: An durable, easily polished igneous rock. Because of its color, grain, and ability to be polished, it's often used inside homes for countertops or on the outside of monumental or civic buildings. However, it can also be used on bridge piers and river walls.
Limestone: A sedimentary rock and the most commonly used rock type for crushed stone in the United States. One of the most versatile rocks for construction, limestone is able to be crushed easily, making it a primary rock used in ready-mix concrete, road construction, and railroads. It is widely available in quarries across the country.
Sandstone: A sedimentary rock used primarily for concrete and masonry work. It is unsuitable for use as a building stone because of its sediment composition.
Slate: A metamorphic rock typically found in layers. Because it is easily mined and cut in these natural layers, it works well in applications requiring thin rock layers. Common examples are roofing tiles, certain types of chalkboards, gravestones, and some pavement applications.
Laterite: A metamorphic rock with a highly porous structure. It is easily quarried in block form and used as a building stone. However, it is important to plaster the surface to eliminate the pores.
Marble: A metamorphic rock. Like granite, it can be polished well and is often used for decorative purposes. Common uses are columns, flooring, statuary, or steps in monumental buildings.
Gneiss: A metamorphic rock. Due to the harmful components of the rock, it is rarely used in construction, although hard varieties are sometimes used in building construction.
Quartzite: A metamorphic rock that is used in building blocks and slabs. It is also used as an aggregate in ready-mix concrete.
As you can see, there are multiple kinds of rocks used for construction. Many of them have a specific purpose for their use in construction, and if you know what to look for, you'll start seeing these rocks in their different applications.