Slate is a metamorphic rock derived from shale. Slate has a dull appearance and occurs in a number of colors including light and dark gray, green, purple and red.


Shale is a sedimentary rock composed of very fine clay particles. Clay forms from the decomposition of the mineral feldspar. Other minerals present in shale are quartz, mica, pyrite, and organic matter. Shale forms in very deep ocean water, lagoons, lakes and swamps where the water is still enough to allow the extremely fine clay and silt particles to settle to the floor. Geologists estimate that shale represents almost ¾ of the sedimentary rock on the Earth’s crust.

Geologists are specific about the definition of the rock called “shale.” Shale is composed of clay particles that are less than 0.004 mm in size.


Sandstone is a sedimentary rock composed mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. Sandstone that contains more than 90% quartz is called quartzose sandstone. When the sandstone contains more than 25% feldspar, it is called arkose or arkosic sandstone. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. The color of sandstone varies, depending on its composition. Argillaceous sandstones are often gray to blue. Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. The most common sandstones have various shades of red, caused by iron oxide (rust). In some instances, there is a purple hue caused by manganese.


Quartzite is a metamorphic rock derived from sandstone that is distinguished from sandstone by its fracture. Sandstone breaks along grain boundaries, whereas quartzite is so well indurated (hardened) that it breaks across constituent grains. The intense heat and pressure of metamorphism causes the quartz grains to compact and become tightly intergrown with each other, resulting in very hard and dense quartzite.


Pumice is a type of extrusive volcanic rock, produced when lava with a very high content of water and gases is discharged from a volcano. As the gas bubbles escape, the lava becomes frothy.  When this lava cools and hardens, the result is a very light rock material filled with tiny bubbles of gas.  Commonly it is light-colored, indicating that it is a volcanic rock high in silica content and low in iron and magnesium, a type usually classed as rhyolite.

Phosphate Rock

The term phosphate rock (or phosphorite) is used to denote any rock with high phosphorus content. The largest and least expensive source of phosphorus is obtained by mining and concentrating phosphate rock from the numerous phosphate deposits of the world.  Some phosphate rock is used to make calcium phosphate nutritional supplements for animals. Pure phosphorus is used to make chemicals for use in industry.  The most important use of phosphate rock, though, is in the production of phosphate fertilizers for agriculture.  Virtually all common fertilizers have an “N-P-K” rating.  Phosphorus is the “P” in fertilizers.  Phosphorus is involved in numerous plant functions, but its most important role is helping plants capture the sun’s energy and begin the photosynthesis process.


Perlite is an amorphous volcanic glass (SiO2) that has relatively high water content, typically formed by the hydration of obsidian. Perlite has the unusual characteristic of expanding and becoming porous when it is heated.  It can expand to as much as twenty times its original volume.  Expansion occurs when the glassy lava rock is heated to 1600 degrees F (871 degrees C) and the water molecules trapped in the rock turn into vapor causing the rock to expand.  (This is the same principle as the water in popcorn that causes the kernel to pop when it is heated.)  Before it is expanded, perlite is commonly gray, but can also be green, brown, blue or red.  After it has been heated, perlite is typically light gray to white.


Obsidian is a black volcanic glass.  It forms from super-cooled lava. It has been used for some of the earliest tools of humankind.


Limestone is a sedimentary rock composed mostly of the mineral calcite and comprising about 15% of the Earth’s sedimentary crust. It is a basic building block of the construction industry (dimension stone) and a chief material from which aggregate, cement, lime and building stone are made. 71% of all crushed stone produced in the U.S. is either limestone or dolomite. As a source for lime, it is used to make paper, plastics, glass, paint, steel, cement, carpets, used in water treatment and purification plants and in the processing of various foods and household items (including medicines).


Granite is the most widespread of igneous rocks, underlying much of the continental crust.  Granite is an intrusive igneous rock. Intrusive rocks form from molten material (magma) that flows and solidifies underground, where magma cools slowly.  Eventually, the overlying rocks are removed, exposing the granite.   Granites usually have a coarse texture (individual minerals are visible without magnification), because the magma cools slowly underground, allowing larger crystal growth.

Granites are most easily characterized as light colored and coarse grained as a result of cooling slowly below the surface. Color variation is a response to the percent of each mineral found in the sample.  The crystals in granite provide a variety of mixed colors — feldspar (pink or red), mica (dark brown or black), quartz (clear pink, white, or black) and amphibole (black).

Granite is high in quartz (about 25%), feldspar, and mica. It is widely used for architectural facades, construction materials, ornamental stone and monuments. Over 40% of dimension stone quarried is granite.  Crushed granite is used as a durable construction material in asphalt and concrete used in highway and infrastructure projects.