Granite and its Geological Classifications

Granite and its Geological Classifications

Geological Classifications

There are three basic classifications for natural stone: igneous, metamorphic and sedimentary.  Igneous rocks are formed from the solidification of minerals from a molten or partially molten state.  Mineral gases and liquids penetrate into the stone and create crystalline formations with various colors.  Metamorphic rock is formed when heat, pressure or chemical reactions alter sedimentary or igneous rocks.  The change may be a development of a crystalline formation, a texture change or a color change.  Sedimentary rocks are formed by the solidification of organic elements such as sediment and plants through heat and pressure.

Dimension stone is a natural building stone that has been cut and finished to specifications.  The most common types of dimension stone are granite, marble, travertine, limestone and slate.


The term granite is used to cover a group of related igneous stones, all of which have their origins deep in the Earth’s molten mantle.  As this extremely hot liquid material rises, cools and solidifies it forms a crystalline, granular structure, hence the name granite.  Granite is made of minerals such as quartz (35%), feldspar (45%) and smaller amounts of mica and hornblendes, which are fused together and is a 6-7 on the Moh’s scale of hardness.

Granite is an excellent choice for interior countertops and floors, exterior cladding and pavement material.  Statistics from various sources indicate that granite quarried in Brazil, India and China comprises approximately two thirds of the granite used worldwide.

Granite has tiny pits, spaces between the various mineral crystals, and fissures, surface cracks.  All granite contain varying amounts of pits and fissures that are visible to the eye, but do not affect the strength or durability of the stone.  Granite can resist scratching from knifes, but can be chipped with enough force.  Granite is typically heat resistant up to temperatures of +/- 480 degrees Fahrenheit; direct, localized heat is still discouraged.  All stones are porous to some degree and should be sealed.  Absorption rates of granite range from 0.05% to 0.40% indicating staining is very slight.


Marble is a metamorphic rock found in the mountainous regions of many countries, most quarried in Italy, Spain and Turkey.  Marble is recrystallized limestone that formed when the limestone softened from heat and pressure and recrystallized into marble.  Marble ranges in color, is usually heavily veined with lots of grain; hardness of marble can range from 3-5 on the Moh’s scale of hardness.

The calcite crystal is the basic building block of true marbles and is vulnerable to damage by mild acids, including those commonly found in a kitchen.  The user, selecting marble for a kitchen, should be aware of and accept the maintenance, patina and scratching with this application.

Onyx – Onyx is often confused with marbles; onyx is sedimentary rock, formed as stalactites and stalagmites in cave interiors.  This formation results in the cryptocrystalline structure that creates translucency in the stone.

Travertine & Limestone

Travertine is a type of limestone; it is actually the terrestrial (land) formed version of limestone, as opposed to the marine based formations of many other limestone’s varieties.  It is formed through the accumulation of calcite from hot springs.  It contains holes that were formed from water flowing through the stone that are filled with resins and cements.  Travertine has a hardness of 4-5 on the Moh’s scale of hardness.

Limestone is a sedimentary rock mainly consisting of calcite containing organic matter and displaying fossils in the material.  Limestone has a smooth granular surface; dense limestone’s can be polished and varies in hardness from 2-7 on the Moh’s scale of hardness.

Travertine and Limestone, like marble, are of a calcium carbonate base, and as such, are vulnerable to damage by exposure to mild acids.  Absorption varies from <1% to >10%, acid sensitivity and high absorption limit the number of suitable applications.

Slate – Slate is a fine-grained metamorphic stone that is formed from clay, sedimentary rock shale and sometimes quartz that hardens by heat and pressure.  Slate can be processed into this sheets and still maintain serviceable strength and rigidity.  Slate can be used for roof shingles to interior flooring.

Soapstone – Soapstone is made of a variety of talc, it is a dense material and is often resist to oxide.  Soapstone is highly heat resistant and traditionally been used for laboratory in chemistry labs.

Types of Surfaces

Honed – provides a flat to low sheen gloss, different levels of gloss can be selected.  This surface is very smooth, but often porous.

Polished – a smooth, glossy surface due to polishing brick and powders used during fabrication.  The reflectivity of polished crystals brings out the brilliant colors and grains of natural stone; the shine comes from the natural reflections of the stone’s crystals, but from a coating.

Flamed – a rough surface that is created by heating the stone crystals, which begin to pop.

Sand Blasted – this surface is the result of a pressurized flow of sand and water that provides a textured surface.

Bush Hammered – a pounding action that develops a textured surface, the degree of roughness can be selected.

As you can see, there are so many granite varieties and so many colors and patterns to choose from (esspecially since no two pieces are the same!). You can narrow down your options by basing your granite choice on the overall theme and style of your room.

Now that you have all of these granite options to choose from, you will be able to select the right granite for your needs and aesthetic.  Whether you choose to go with granite kitchen countertops or a granite bathroom vanity, you will surely enjoy the beauty and functionality of granite.

Denver, Colorado