STONE
Stone refers to natural rocks after their removal from the earth crust. The familiar stone types that used today are identified through four categories for remodeling which are sedimentary, metamorphic, igneous and man-made. Classification depends on the process by which they were produced within or on the earth's surface.

Sedimentary rocks are produced by the weathering and erosion of older rocks. In the earliest geological time, these would have been the original igneous rocks but subsequently other sedimentary and metamorphic rocks too will have been reworked. Weathering action by water, ice and wind breaks the rocks down into small fragments which are then carried by rivers and sorted into size and nature by further water action. Most deposits are laid down in the oceans as sedimentary beds of mud or sand, which build up in layers, becomes compressed and eventually are cemented together by minerals. They were bonded through millions of years of heat and pressure.

Sedimentary rocks

For thousands of years, little pieces of our earth have been eroded, broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of the other. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

While for the metamorphic stone, it is originates from a natural change from one type of stone to another type through the mixture of heat, minerals and pressure. The change may be development of a crystalline formation, a texture change, or a colour change. Clay is metamorphosed to slate, limestone to marble and sandstone to quartzite.

metamorphic stone

Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.

Igneous rocks are the oldest, having been formed by the solidification of the molten core of the earth or magma. They form about 95% of the earth crust which is up to 16km thick. Igneous stones are mainly formed through volcanic material such as magma. Underneath the earth surface, liquid magma cooled and solidified. Mineral gases and liquids penetrated into the stone and created new crystalline formations with various colours. If the magma stays underground, cools slowly and makes big crystals, it is called intrusive igneous rock. Extrusive igneous rock is made when magma comes to the Earth’s surface in the form of lava and then cools and hardens. Since lava cools quicker, the crystals of surface igneous rock are smaller. Solidification slowly within the earth crust produces plutonic rocks while solidification rapidly at the surface produces volcanic rocks. Slow cooling of plutonic rocks allowed large crystals to grow which are characteristic of granites. Volcanic rocks such as pumice (grey volcanic rocks) basalt (dark volcanic rocks) are fine-grained and individual crystals cannot be distinguished by the eye.

Igneous rocks

Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

The last type of stone which familiar use nowadays is man-made stone. Man-made stones are derived of unnatural mixtures such as resin or cement with the additive of stone chips.

The example of sedimentary stones is limestone, sandstone, and fossil stone. Limestone mainly consists of calcium carbonate, either crystallised from solution as calcite or formed from accumulations of fossilised shells deposited by various sea organisms. There are various type of limestones such as oolitic limestone, organic limestone, crystallised limestone, dolomitic limestone. It does not show much graining or crystalline structure. It has a smooth granular surface. Varies in hardness. Some dense limestone can be polished. Common colours of limestone are black, gray, white, yellow and brown. It is more likely to stain than marble. Limestone is know to contain lime from sea water. Sandstone is a very durable formation of quartz grains ( sand ). It usually formed in light brown or red colours. Categorized by the most popular and stone bonding agents such as calcium carbonate, silica, iron oxide and dolomite produce calcareous, siliceous, ferruginous and dolomitic sandstones. Depending upon the nature of the original sand deposit, the sandstones may be fine or coarse in texture. Sandstones are generally frost-resistant. Fossil stone considered a limestone that contains natural fossil such as sea shells and plants. It usually a cream or reddish colour. Fossil stone is formed through the accumulation of calcite from hot springs. It contains lots of holes that were formed from water flowing through the stone. These holes are often filled with synthetic resins or cement. 

Marble, slate, quartzite are metamorphic stone. Marble is metamorphosed limestone in which the calcium carbonate has been recrystallised into a mosaic of approximately equal-sized calcite crystals. It is a recrysrallized limestone that formed when the limestone softened from heat and pressure and crystallized into marble where mineral changes occurred. Some limestones which can be polished are sold as marble but true marble will not contain any fossilised remains. Marble is attacked by acids. The main consistency is calcium and dolomite. Ranges in many colours and is usually heavily veined and shows lots of grains. The hardness rate of marble is from 2.5 to 5 MOH scale. Marble is classified into three categories: Dolomite ( if it has more than 40% magnesium carbonate), Magnesian ( if it has between 5% and 40% magnesium carbonate) , and Calcite ( if it has less than 5% magnesium carbonate ). Slate is derived from fine-grained sand-free claysediments. Slate is strong, acid- and frost-resistant, lasting up to 400 years as a roofing material. Quartzite is also metamorphosed sandstone. The grains of quartz are recrystallised into a matrix of quartz, producing a durable and very hard wearing stone used mainly as a flooring material.

Granite and basalt are igneous stone. Granite primarily made of Quartz, Feldspar, and Potassium. Colours of granite include grey, pink, black, blue, green, red, yellow, brown and contain very little calcite. It provides a heavy crystalline and granular appearance with mineral grains. It is very hard material and easier to maintain than marble. Because most of tvhe granites are hard and dense and thus form highly durable building materials, virtually impermeable to water, resistant to impact damage and stable within industrial environments. Yet, it is still porous and will stain. There are different types of granite depending on the percentage mix quartz, mica and feldspar. Black granite is known as an Anorthosite. It contains very little quartz and feldspar and also has a different composition that true granite. Granite is available for flooring and for hard landscaping including pavings and kerbs. Polished granite is also used as a kitchen countertop material due to its strength, durability and high- quality finish. Basalt is a fine-grained stone nearly as hard as granite. It can be melted at 2400^o C and cast into tile units which are deep steel grey in colour. Annealing in a furnace produces a hard virtually maintenance-free shiny textured surface.

Man made stone include Terrazzo, Agglomerate or Conglomerate and Cultured or Faux Marble. Terrazzo is marble and granite chips embedded in cement composition while Agglomerate or Conglomerate is marble chips embedded in coloured resin composition. Cultured or Faux Marble is a mix of resin that are painted or mixed with a paint to look like marble which consider a man-made stone.

There are some main agencies which causes the deterioration of stone such as soluble salt action, atmospheric pollution, frost, corrosion of metal components, poor design or workmanship. 

Soluble salt action

If moisture containing soluble salts evaporates from the surface of stonework, then the salts will be left either on the surface as white efflorescence or as crystals within the porous surface layer If the wetting and drying cycles continue, the crystalline material builds up within the pores to the point at which the pressure produced exceed the tensile strength of the stone, causing it to crumble. The more porous stones such as limestone and sandstone, are susceptible to soluble-salt action.

Atmospheric pollution

Stones based on calcium carbonate are particularly vulnerable to attack by acid atmospheric pollutants. Sulfur dioxide in the presence of water and oxygen from the air produces sulfuric acid which attacks calcium carbonate to produce calcium sulfate. Limestone and calcareous sandstones are vulnerable to attack.

Frost

Frost damage occurs in the parts of a building which become frozen when wet. Frost causes the separation of pieces of stone but it does not produce powder as in crystallisation attack. Generally, limestone is more vulnerable to frost damage than sandstones. Marble, slate and granite used in a building are normally unaffected by frost due to their low porosities.

Corrosion of metal components

Rainwater run-off from copper and its alloys can cause green colour staining on limestones. Iron & steel produce rust staining which is difficult to remove from porous stones. Considerable damage is caused by the expansion of iron and steel in stonework caused by corrosion. All new and replacement fixings should normally be manufactured from stainless steel or non-ferrous or non-ferrous metals.

Fire rarely causes the complete destruction of stonework. In the case of granite, marble and most sandstones, the surfaces may be blackened. Limestones are generally unaffected by fire, although the paler colours may turn permanently pink due to the oxidation of iron oxides within the stone. 

Generally, large plants should be removed from old stonework. However, creeper and similar species are not considered harmful.

External granite, marble and slate claddings require regular washing with a mild detergent solution; in particular, highly polished external marble should be washed at least twice per year to prevent dulling of the surface. Limestone, which is not self-cleaned by rainwater, should be cleaned with a fine water spray and brushing, removing deposit from the surface. Sandstone is usually cleaned mechanically by abrasive blasting or chemical cleaning.

Stone is preserve by coatings such as silicone water repellents should only be applied to stonework following expert advice and testing. Silicone treatment may in certain cases cause a build-up of salt deposits behind the treated layer, eventually causing failure. Silicone treatment should not be applied to already decayed stone surfaces.