Gravel is small rock, produced by crushing big stones cut from quarries into smaller pieces or created naturally by some water like a creek or river. What many people think of as gravel is actually crushed limestone, ground into rocks of various diameters and sorted by size. It is used as a base for concrete and other construction and also is frequently used as paving.
Limestone gravel makes a good driveway or walkway because it can be compacted into a solid, durable surface. Paving limestone is small, usually about 1/2-inch in diameter. It usually is mixed with limestone dust, which fills the gaps between the irregular surfaces of the gravel pebbles to make a more solid base. A limestone driveway or walkway will last for years, with little maintenance unless it is subjected to heavy water runoff. Limestone's big drawback is that it produces dust.
Creek gravel is just that -- solid rocks worn down by water and dug from the bed of a creek, river or shoreline. It varies widely in color, depending on the type of rock washed into the waterway. Creek gravel rocks also vary widely in size, up to about 2 inches, and in shape, with no regular form. Creek gravel is often used for driveway paving in rural areas and for walkways in gardens.
Cobble is another water-washed gravel, but composed of stones with rounded surfaces. Cobble comes in all sizes, from about 1/2-inch in diameter up to the large rocks used for years in cobblestone streets and sidewalks. Cobble has a wide range of colors and shapes, depending on the locale of its origin. Small cobble makes an excellent walkway paving because it can be compacted like limestone, but is a less solid surface and does not create dust. Cobble's disadvantage for a driveway is that its rounded surfaces do not bind together as firmly as irregular limestone.
Pea gravel is a very fine water-washed gravel, with individual rocks no bigger than 3/8-inch in diameter and usually smaller. Its colors vary widely, from light tans to deep browns, depending on the source of the rock that was eventually washed down to pea size. It makes a colorful and attractive walkway and also is frequently used for driveways. Its disadvantage for driveways is that while it compacts firmly, the rounded stones do not form a solid surface, so vehicles can create tracks in it.
Sea Shell Gravel
Some coastal regions produce gravel made of sea shells. Shells frequently are crushed like limestone and produce paving gravel quite similar to limestone. The two materials are chemically very close. Shell gravel makes good walkways and driveways, solid like compacted limestone, but with less stone dust than limestone. Sea gravel is usually not found far from such regions as the Gulf Coast, which have an abundance of shells.