3. Marble
Marble is a type of metamorphic rock which is characterized by swirls of color from impurities and the ability to take a very high polish. This stone has been used for thousands of years in art and architecture, and it continues to be very popular today in applications ranging from garden paving stones to mantles.
A variety of metamorphic rocks including granite are sometimes called “marble,” but true marble comes from either limestone or dolomite. As these sedimentary rocks undergo heat and pressure, they recrystallize, forming marble. Marble often has no clearly discernible grain, and it is typically streaked with veins and clouds of impurity which vary widely in color. When cut into slabs, these impurities can create striking and fanciful designs.
Pure white marble comes from limestone with no impurities, and it has historically been highly prized. In many cultures, white marble is a sign of purity, and it is has been associated with good fortune in education and finance, which is why many colleges and financial institutions have buildings which feature a lot of white marble. Marble can also be black, gray, pink, or green, and it may be streaked with many variations of these colors..
4. Quartz
Quartz is a hard crystalline mineral which is found abundantly all over the Earth in a variety of forms. It is the most common mineral on Earth, making up 12% of the Earth's crust by volume, and is used in a wide variety of applications including jewelry, scientific research, manufacturing, and building. Because of its many uses, quartz is also a very important mineral, and some rare varieties of quartz are considered to be quite valuable.
Silicon dioxide is the proper chemical name for quartz, which acquires different colors and properties depending on how it was formed. Commonly, quartz takes on impurities which cause it to be colored, as is the case with the quartz forms amethyst, citrine, and rose quartz. In these instances, the crystalline structure of the quartz is readily visible, and the stone can sometimes form very large clusters of distinctive crystals. These quartzes also tend to be clear, although they may have small inclusions or areas of fracture.
In other instances, the quartz crystals are too small to see with the naked eye. This form of quartz is opaque, and sometimes called milky quartz or chalcedony. Chalcedony is quartz which has formed through deposits of layers of colored material, such as agate, jasper, and carnelian. Quartz can also appear mixed in with other geologic material, as is the case with granite and sandstone. Because quartz does not dissolve readily in water, concentrations of it tend to increase as other rocks wear away, living mixed rocks with a high percentage of quartz in their composition.
