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Geology and Natural Stone Detection


Those in the stone trade, and those fascinated in stone, should be familiar with basic geology.  As a doctor studies the systems of the human body to advise treatments and remedies, you should also be familiar with the formations and changes of stone. The study of these changes is known as geology.

Why is geology so imperative? There are many reasons. Granite and marble, for example, differ greatly in their hardness. A hard material such as granite may require harder abrasives for refinishing. Certain limestones contain very pointed minerals that can quickly spoil a polishing pad or set of diamond abrasives. You need to know which stones these are.

The idea of the following is to provide you with a basic introduction to geology. In order to understand the problems you may encounter with stone, it is vital that you understand the formations and changes that occur with stone. A thorough material consideration will make your job much easier.

Stone Types

It is understood there are somewhere near 9,000 different types of stone. Of course it would be unworkable for anyone to know every stone, but all stone can be classified into three groups. Learning these three categories and their type can solve most of the problems you may face. For example: Granite, which is an igneous rock, contains quartz, which is hard. For this reason, the proper abrasive must be chosen for finishing. Marble, which is a metamorphic rock, contains calcium carbonate that reacts to acid. This awareness would tell you that acidic cleaners would spoil most polished marble surfaces. Travertine, a sedimentary stone, consists of small grains of minerals bonded to each other with softer minerals. The minerals are very coarse and can quickly spoil a diamond abrasive. The following are the three classifications of stone and how to recognize them.

Igneous Rocks (Granite)

Igneous rocks are formed from the solidification of magma far down in the earth. They contain 45 to 66 percent silica quartz. The residual minerals are mostly feldspar, mica and iron ores. Granite is the most abundant igneous rock found on Earth.

Characteristics of Igneous Rocks

Rocks exhibit a crystalline form with grain size ranging from small to several inches. The large crystal granites are formed when the magma cools slowly. The smaller crystals are formed with speedy cooling. All this takes place before the magma reaches the Earth’s surface. When, and if, it reaches the surface, it’s called lava.

Hardness ranges from six and higher on the Mohs’ scale. Igneous rocks can be recognized without difficulty by performing a scratch test with an ordinary knife blade. If it is hard to scratch, it is most likely igneous.

Igneous rocks will generally not react with acids. However Hydrofluoric Acid (HF) will react with a polished surface. Many stone cleaners designed for sandstone contain HF. Make sure to read product labels and material safety data sheets when using any stone cleaner or chemical. Keep away from using HF on all stones if possible.

The minerals contained in igneous rocks are typically dense and packed tightly.

They lack bedding or foliation. On the other hand, be aware of a granite look-a-like known as gneiss. Gneiss looks like an igneous rock but is a transitional material between metamorphic and igneous. It is a brittle material and does have a bedding plane.

Colour key for igneous rocks:

White granites are wealthy in potassium.

Combinations of mica and quartz result in gray.

Pink contains sodium and calcium rich feldspar.

Sedimentary Rocks (Limestone’s and Sandstones)

Sedimentary rocks are produced from erosion of other rocks, compression and underground water erosion. The sedimentary rocks cover nearly 75 percent of the Earth’s surface. For the purposes of defining decorative stone, sedimentary rocks can be classified into two types: limestones and sandstones.

Limestones are formed in shallow waters of the sea shelf. They consist of calcite, but maybe mixed with other minerals depending on the water clarity when it was formed. A lot of limestones contain a mineral known as magnesium carbonate dolomite.

Dolomite does not react with acid if not it is first crushed. Any rock with more than 50 percent carbonate mineral is classified as a limestone. Common limestones are travertine, slate, coral, shell stone or coquina. Limestones have a bedding plane, and because they are formed in the sea, they often contain fossils and shell fragments that make them more simply identifiable.

Sandstones are also sedimentary rocks that are primarily quartz minerals insecurely cemented together with calcite, iron oxides and/or mud. Sandstones are frequently used as a dimensional building stone. Major cities throughout the world contain buildings made with sandstone. Some common sandstone is brown stone and shale. Sandstone can be recognized by its distinct sand-like appearance. It fractures simply around the individual grains, and exhibits a distinct bedding plane.

Metamorphic Rocks Marble And Slate

Metamorphic rocks are limestones that have been open to high temperature and pressure over a long period of time. This change is known as metamorphosis and causes the minerals to go through a molten phase. For this reason, most marbles have distinct veins, swirls or bands, but there are exceptions. Light veins may contain quartz and feldspar, while dark veins may contain hornblend or biotite. All marbles will have calcite and/or dolomite and will react with acid.

Mineral Identification

In order to identify the varieties of stone you may come across, you need to be able to recognize the minerals in the rocks. Certain groups of rocks contain specific types of minerals. We know granites are igneous and include quartz, marble is metamorphic and contains calcite, and sedimentary materials such as limestone can contain dolomite.

The minerals contained in the stones we use have some physical and chemical properties. Some of these properties can be tested and will tell us which mineral we are dealing with. The following are those properties:

Luster :— Luster is the way a mineral reflects light from its surface. A mineral can be metallic, which means it looks glossy, or non-metallic, which has no glossy reflection but is described as glassy or vitreous, pearly, silky, dull, greasy or soapy.

Hardness :— Frederick Mobs in 1822 developed the rigidity test, which measures how defiant a mineral is to abrasion. We commonly call this the scratch test. Taking minerals and scratching one against the other was how the test was developed. The one that scratched was softer then the one that caused the scratch. This test was continued until 10 minerals were discovered. The minerals were arranged in order of rigidity and given a number. 1 was the softest and 10 was the hardest.

Cleavage : — Cleavage is how the mineral breaks or fractures. Some minerals, when they break, will have one, two or three cleavages. This is perhaps the hardest property to determine and will require the use of a field microscope or hand lens.

Colour : — Colour is a significant clue to identifying certain minerals. Stone gets its colour from many of these minerals. Some minerals only have to be present in small quantities to lend colour to a stone. For example, a lot of the red granites and marbles are the result of the mineral iron.

Acid Test : — The acid test is a significant test and can straight away tell you if you are dealing with a calcium-based stone. A simple drop of a dilute solution of hydrochloric acid will tell you if you have marble, limestone or granite. If the acid bubbles and/or fizzes, you have calcium. No reaction means there is no calcium and you may have granite or possibly a dolomite limestone.

Those are the most general properties used to recognize a mineral. Geologists and others may also use specific gravity; magnetism, electrical properties, optics and taste for further identification.

Other Stone Types

Marble, granite and limestone are the most common interior stone types that you will encounter. However, there are others. The following is a brief explanation of those types:

Slate : –  Slate can be found both inside and outside. It is a stone that has been metamorphosed from shale and consists of clay-like materials. It can be identified by its sheet looking appearance and is found in earth-tone colours. Slate will hardly ever have a high polish except if a coating is put on it. Slate can be a predicament for the architect/designer. It contains high clay content, so it will often flake, spall and develop efflorescence with no trouble. This is especially true in wet areas. Slate is treated with a quality silicone penetrating sealer.

Sandstone : – Sandstone is a sedimentary material, which is made up of sand crystals cemented together with natural clays. It is hardly ever seen as flooring material, but an architect or planner will run across a floor or two. Sandstone is very coarse because of its sandy structure and is very abrasive on diamond discs. It is acid-resistant and hardly ever polished. Sandstone is very permeable and should be sealed with a penetrating sealer.

Quartzite : – Quartzite is also a rare flooring material but is gaining in reputation. It is metamorphosed quartz sand and is classified as sandstone, exhibiting those properties.

Shell Stone or Coquina : – Shell stone or Coquina is a limestone composed of broken fragments of shell and coral. It is a permeable sedimentary material and is simply identified by its shell or coral fragments. It should not be resurfaced because it is an abrasive stone and will tear diamond discs.

Flagstone/Blue Stone : – Flagstone is a term given to almost all material cut into thin, irregular shapes. Found broadly on pavments, foyer entrances and pool decks, flagstone can be almost any stone material but is commonly the sandstone type known as blue stone. Blue stone gets its name from its distinctive blue colour. Flagstone should not be ground and should only be cleaned and sealed.

Onyx : – Onyx is a type of marble, which has been created by the deposits of cold solutions. It is a translucent material with veins running concentrically to one another. It is costly and can be found on tabletops and small pieces of furniture. It can be treated just like marble and takes a high gloss.

Soapstone : – Soapstone is one of the softest materials composed of talc. For this cause it makes an excellent carving material and can be found on fireplace surrounds and hearths. It is treated like marble and should be sealed.

Travertine : – Travertine is a limestone formed in hot springs. Water movement causes the erosion of the stone, creating numerous holes of altering sizes. A polished travertine will have its holes filled with a colour-matched cement fill. These fillers do not take a high gloss and therefore the overall polish is marked.

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