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Types of Rocks


When limestone, a sedimentary rock, gets buried deep in the earth for millions of years, the heat and pressure can change it into a metamorphic rock called marble. Marble is strong and can be polished to a beautiful luster. It is widely used for buildings and statues. As you can see from the picture, marble has a very sparkly luster. It occurs in many different colors such as white, pink, gray, red, yellow or black. 


Basalt is a hard, black volcanic rock. Basalt is the most common rock type in the Earth's crust. 
Depending on how it is erupted, basalt can be hard and massive (Figure 1) or crumbly and full of bubbles (Figure 2).
Figure 1.
Figure 2.


Slate is a metamorphic rock that is formed when shales and clays are put under great pressure and heated inside the earth for millions of years. Like shale, it splits apart into sheets, which means that it has good cleavage. Slate is usually gray or black and is used to make blackboards and roofing tiles.



Limestone is a sedimentary rock. It is formed by layers of small pieces of rock and stones pressed hard together. The stone forms in wet areas which mean it will also contain of shells and waste matter from organisms that live in water.Limestone caves are formed when rainwater seeps through cracks in limestone rock and dissolves it. Limestone turns into the metamorphic rock marble when it is subjected to high amounts of pressure and heat.

 Uses of limestone
Limestone is important to making cement.
It is used in glass making. 
Layers of crushed limestone are placed underneath roads and underneath railroad tracks. 
It helps clean drinking water and treat sewage. 
Fertilizer used by farmers contains ground up limestone. It is a food for the plants and is good for the soil.
Animal feed also contains some limestone.


Gneiss is a type of metamorphic rock that forms when a sedimentary or igneous rock is exposed to extreme temperatures and pressure. When this happens, virtually no traces of the original rock remain. Gneiss rocks are characterized by their arrangement of minerals into long bands. Gneiss is distinctive among other rocks that have bands because its minerals are not evenly distributed so the bands are various widths.
Gneiss has also been used to construct buildings and gravestones.


Conglomerate rock is a type of sedimentary rock. It composed of large, rounded rock fragments contained within a smaller grained matrix of sediments. The large fragments, known as clasts, can come in a range of sizes, but all must be greater than two milimeters in diameter in order for the rock to be classified as a conglomerate.

Uses for Conglomerate

Though interesting to look at, conglomerate is not the world's most useful rock type. It lacks the glamour of granite or the strength of slate. But it does do one thing very well - being itself. Remember how much it looks like a concrete aggregate driveway? Well, conglomerate can actually be used in the making of concrete. 


Shale rocks are those that are made of clay-sized particles and are have a laminated appearance. They are a type of sedimentary rock. Shale is the abundant rock found on Earth. They are usually found in areas where gentle waters have deposited sediments that become compacted together. Shale usually forms in sheets. Clay is a key component in shale rock. After formation, shale is usually deposited into lakes and rivers by slow moving water. Shale that is exposed to extreme heat and pressure can change into slate.


Sandstone is a type of sedimentary rock. It forms when grains of sand are compacted together over very long periods of time. Normally this sand has an abundance of quartz but can also contain other minerals and materials. Sandstone comes in a variety of colors including red, yellow, gray, and brown. Sandstone can form under the sea or on land.

Sandstone has been used to construct buildings, statues, and fountains.


Granite is an igneous rock composed of mostly two minerals: quartz and feldspar. It is an intrusive rock, meaning that it crystallized from magma that cooled far below the earth's surface. Granites usually have a coarse texture (individual minerals are visible without magnification), because the magma cools slowly underground, allowing larger crystal growth. It is the most common igneous rock.
Granite is used in construction, for everything from basic building to beautiful polished countertops. 


Obsidian rock are igneous rocks that form when lava cools quickly above ground. The result is a rock that cooled so fast, crystals did not get a chance to form. Obsidian is a volcanic glass with a smooth and uniform structure. Obsidian can only form near active volcanoes.
Obsidian is commonly used during surgical procedures because it is often sharper than traditional surgical tools.
Obsidian is used as a gemstone in jewelry. 


Pumice is an extrusive igneous rock formed as a result of volcanic eruptions. Extrusive means that it forms outside of the volcano (as opposed to inside the volcano in the magma chamber), and as a result, the magma cools quickly after exiting the volcano. Little pockets of air can be seen in them. This rock is so light, that many pumice rocks will actually float in water.  Pumice is actually a kind of glass and not a mixture of minerals.  

It is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in soaps.


Schist is a metamorphic rock that contains flakes of mica. Schist can originate as a sedimentary rock or igneous rock. Large crystals cause schist rocks to reflect lots of light which gives it a very shiny appearance.  It is made up of clay minerals.