Metamorphic rock was formed by extreme pressure and high temperature, making it harder than it was before. Either igneous rock or sedimentary rock can be changed by extreme pressure and high temperature. During the metamorphic process the crystals in the rock change size, known as recrystallization. For example, small calcite crystals in limestone become larger when they transform into marble.
Marble: Marble has always been considered a highly desirable material for its translucent look and soft colors. It is derived from limestone and is commonly used for floors, furniture tops, and countertops. Marble’s main drawbacks are that it can be etched by acidic foods, and it is porous (absorbs oil, water, and stains). Marble usually has colored veins running through it; the different colors come from different minerals. Red-yellow comes from iron oxides, blacks and blue-grays from bitumens, and green from silicate chlorite and mica. Used for floors, walls, furniture tops, and exteriors, marble is usually highly polished to show off the colors, but it will also show scratches. Marble is thought of as a product of Italy, but there are large amounts in the United States. There are quarries of marble in Georgia producing several colors of marble. Also, there is a large quarry of white marble in western Colorado that supplied the marble for the Lincoln Monument and Arlington Cemetery. Currently, the quarry in Marble, Colorado, produces white marble shipped to Italy for the European market because it produces larger pieces of marble than Italian quarries.
Quartzite: Quartzite is recrystallized sandstone. It is usually white or gray, but may have pink or orange colors when iron is present. Quartzite is a very hard stone used for road construction.
Slate: This is a dense, fine-grained rock formed by compression of sedimentary rock shale. Common colors are gray to black, but some green, brown, and red are available. Slate is found in Great Britain, western Europe, eastern United States, Brazil, India, and China. Its natural finish has textural variations known as cleft face. Slate can be finished as either honed for a dull finish or rubbed to produce a smoother finish. Slate is commonly used for walkways today, but in the past, slate was used as roofing shingles.
Soapstone: This metamorphic rock is very soft, so it does not take a polish, but it is dense and durable. It contains talc and other minerals. Soapstone is available in slabs up to 5 feet long. It does not react to heat, making it safe for countertops, fireplace hearths, and steps. It is nonporous, which is good for water and resisting staining. Soapstone starts as a light bluish-gray and is usually treated with oil weekly, which gives it a darker appearance over time.
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Stone 71 72 Decoration Slabs of Granite, Marble, and Other Stones on Display in the Las Vegas Warehouse for Designers to Select for Projects. Image used with permission of Dal-Tile Distribution, Inc. Stone is extracted from the earth where a large amount of rock is found. In its natural setting, it is a rock, but after it is extracted from the quarry, it is called stone. The area where the rock is found becomes the quarry, and the stone extracted is named for the location of the quarry. What we consider stone color and pattern names are really the name of the location where that stone was quarried. For example, Carrara marble was quarried in Carrara, Italy. Similarly, Cottonwood limestone was originally found in Cottonwood Falls, Kansas, and is quarried in that general area. Stone is extracted by drilling long, narrow, close-together holes into the stone. These long vertical holes perforate the stone so that it breaks along the line of holes. After a large piece is removed from the quarry, it is cut with a diamond-studded saw into manageable blocks or slabs. The slabs are numbered with spray paint as they are removed to a stacking area. A water-jet cutting machine cuts the stone into the desired size pieces. The machine can cut angles less than one degree, which are for intricate inlay patterns. Designers must consider more than the appearance of color, texture, and grain size (petrographical) when selecting a block of stone to be cut. The cut direction affects the strength of the stone. Stone can be cut from the side of a block, the top of the block, or the end of a block. Advances in technology have allowed for thinner slabs to be produced. Currently, technology can produce slabs as thin as 4 millimeters. These thin slabs are very fragile and need to have backing added for strength. Types of backing are aluminum honeycomb and fiberglass and epoxy resin. Finishing When a stone slab is moved to a fabrication mill its finish is decided. Polishing has been a traditionally desirable finish because polishing brings the variety of colors in the stone structure to light. Polished stone does not show saw or grinding marks. It is the easiest finish to clean, but it is also the slickest a drawback for floors. A flamed finish can be used on stones containing quartz. Flaming creates a rough texture with overlapping shadow lines that is nonslip but also hard to clean. The heat from flaming causes color changes. Here is a list of different types of finishes used for stone. Quarry-face or sawn-face shows marks of removal from the quarry. Hand finishes can be chiseled to produce a texture.
Machine finishes can be polished, which is often described as mirror finish or high gloss; honed, which is a dull sheen; or flamed, which is a rougher finish.
Sandblasting uses an abrasive to give a coarsely or finely stippled finish.
Tumbled finish is the result of placing small pieces or tiles of stone into a barrel with abrasive grit and water. The barrel slowly rotates to texture the stone.
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