Basic Knowledge of Stones

Different types of stones are classified according to their mineral content and the process of their formation. Good stones are characterized by durability and hardness, strength, amenability to dressing, appearance, weight, fineness of grain and compactness and porosity and absorption. In order to be suitable as building stone, a rock should have specific qualities such as capacity to stand the ravages of time and weather, requisite strength to bear strain and super-incumbent weight and attractive colour' and general appearance. Its structure must also be such as to allow quarrying into good sized blocks or planes. It should possess inherent features such as joints and (in sedimentary rocks) bedding planes.

India possesses extensive deposits of different kinds of building and monumental stones. It is one of the few major countries known for the production and export of granites and marbles of various colours, besides slate and other building stones. The most important building stones in India are the granites/and allied rocks, the standstones, limestones and marbles, and the laterites.

Types of Stones
The different types of stone are:

Igneous Stone
Igneous stones are formed from molten or partly molten material i.e. magma, through solidification of magna. Lava is a form of magma cools and solidifies on the surface of the earth. Liquid magma cools and solidifies underneath the Earth's surface, and then mineral gases and liquids penetrate the stone and create new crystalline formations and various colors.

Of all igneous rocks, Granite is the most common choice. It is a coarsely crystalline unstratified igneous rock composed of alkali feldspars, quartz, mica and hornblende. Since granite is highly speckled, it varies greatly in color depending on the range of the minerals present and the location of the quarry. When the mica in granite is white, it is known as muscovite, when it is dark it is biolite. In certain types of granite, hornblende is an important ingredient and it may be even more abundant than the micas. Due to its chemistry, granite is a much harder stone. It doesn't scratch as easily and rarely needs heavy-duty grinding.

Sedimentary Stone
Sedimentary stones are layered rocks, formed through the accumulation and solidification of sediments, which may originally be made up of minerals, rock debris, or animal or vegetable matter. These stones come from such organic elements as glaciers, rivers, wind, oceans and plants. Tiny sedimentary pieces break off from these elements and accumulate to form rock beds. They are bonded through millions of years of heat and pressure and include limestone, sandstone and travertine. The different types of sedimentary stones are:

• Some stone types are the result of some pre-existing rock's demise through the weathering process. Sandstone "Siliceous"
• The biogenic stones are produced by living organisms such as marine creatures or vegetation which eventually died and the remains became part of the sediment on the bottom of ancient sea beds. Certain Limestone's, Coralstones, Fossilstone, Shellstone "Calcareous".
• The third is produced by inorganic chemical material being deposited on the floor of ancient sea, lake beds and springs through the process of precipitation or evaporation. The white fur deposits in kettles in which hard water has been boiled is a good example of this type of stone. Certain Limestone's and all Travertine's "Calcareous"
• The last sedimentary category are produced as a result of volcanic activity. This stone is composed of volcanic ash with a variety of secondary older rock materials from the surrounding area, volcanic and igneous in nature. This category is considered a tuff sedimentary stone. It is also classified as a igneous stone due to its volcanic origin. Cantera and Adoquin are of this variety "Siliceous"

Metamorphic Stones
Metamorphic stones are formed in some fashion from a pre-existing rock, through heat, pressure, the effect of superheated fluids, or any combination of these forces. The change can be a development of crystalline formation, a texture change or even a color change.

Metamorphic is derived from the Greek word "meta" denoting a change and "morph" meaning form or a change in form. This changing of one rock type into another new type takes place by an increase in temperature or pressure or a combination of both. The original minerals of the stone recrystallize, small crystals merge to form larger crystals with no changes in the mineral chemistry, fine-grained calcite in limestone recrystallizes to a coarse-grained calcite crystal structure in marble; or there may by a transformation into a different set of metamorphic minerals. This recrystallization transforms the Shale into a colorful Slate "Siliceous". Limestone becomes a multi colored Marble "Calcareous" and the grainy Sandstone alters into a highly crystallized sparkling Quartzite "Siliceous".