The rock forming the earth’s crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning ‘changed in form’ usually into more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism.

Granites usually have been classes as igneous rocks derived from molten masses or magmas, but there is wide evidence that the origin of some granites may be attributed to regional metamorphism or preexisting rocks, rearrangement and recrystallization taking place without a liquid or molten stage.

The chief materials of which granites are formed are feldspars and quartz and smaller amounts of mica and hornblende. They are classified as fine-grained, medium grained, or coarse-grained. Medium-grained granites are those in which the feldspar crystals average about ¼-inch (6mm) in diameter. If relatively coarse grained crystals appear in fine-grained groundmass the rock is designated as porphyritic granite. A rock may have the mineral constituents of a granite but may show a banded or platy structure owing to recrystallization, folding, or other changes while the rock was in plastic or semi-molten condition. Such metamorphic rocks are called granite gneisses.

The rock forming the earth’s crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning ‘changed in form’ usually into more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism. Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many Constituent minerals may be dissolved, transported, and reprecipitated by thermal waters. Heat and pressure may cause recrystallization

Limestone is defined as a rock of sedimentary origin composed principally of calcium carbonate or the double carbonate of calcium and magnesium, or a combination of these two minerals.

The rock forming the earth’s crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning ‘changed in form’ usually into more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism. Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many Constituent minerals may be dissolved, transported, and reprecipitated by thermal waters. Heat and pressure may cause recrystallization.

Marble is, therefore, metamorphic rock resulting from the recrystallization of limestone. Commercially, however: all calcareous rocks produced by nature and capable of taking a polished are called marble, as are some dolomitic and serpentine rocks.

Variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos.

The rock forming the earth’s crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning ‘changed in form’ usually into more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism. Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many Constituent minerals may be dissolved, transported, and reprecipitated by thermal waters. Heat and pressure may cause recrystallization.

In this way, new rocks are formed, differing widely from the igneous or sedimentary types, and usually much harder than with. Thus shales and related rocks may be altered into slate. The shales from which slates originate were deposited previously as clay beds. There beds of shale, at first horizontal, were tilted by subsequent earth movements, and the intense metamorphism that converted these into slates folded and contracted them. Slate, then, belongs to the metamorphic group of rocks derived from clays and shales and possessing a cleavage that permits it to be split into thin sheets.

Soapstone is a metamorphosed, easily worked igneous stone characterized by a "soapy" feeling when touched. Colors range from dark gray to bluish or greenish gray. Its heat retention qualities make it an ideal cladding for free-standing coal or wood-fired room heaters. Soapstone is also chemical, stain and weather-resistant, and is useful for sinks and labratory tops as well as general building purposes.

Form of calcium carbonate, resulting from deposition by springs or rivers. It is often beautifully colored and banded as a result of the presence of iron compounds or other (e.g., organic) impurities. This material is variously known as calc-sinter and calcareous tufa. Travertine is generally less coarse-grained and takes a higher polish than stalactite and stalagmite, which are similar in composition and origin.