Geology of Stone

Granite is an Igneous rock. Igneous rocks are found when magma (molten rock from beneath the earth's crust) cools and solidifies. There are two different types of Igneous Rocks, intrusive and extrusive.

Intrusive is when the igneous rock is formed in cracks and holes within the earth's surface, the magma cools slowly and forms coarse grained rocks such as granites.

Extrusive is when lava (magma) is ejected from the earth's crust (through volcanic eruption) and cools rapidly producing more fine grained rocks such as basalt (Bluestone).

Marble is a Metamorphic rock. Metamorphic rocks occur as igneous, sedimentary, other metamorphic rocks are changed by extreme heat (contact metamorphism) or extremely high pressure (regional metamorphism). Contact metamorphism can occur during volcanic eruption where regional metamorphism can occur during an earthquake, (folding of a mountain range). Metamorphism produces marbles and slates.

Sandstone is a Sedimentary rock. Sedimentary rocks are formed by consolidation of accumulated sediments. Sandstone is a Clastic Sedimentary rock which is formed from other rocks being broken down into fragments by weathering over time then being transported and deposited elsewhere. Once these fragments have been deposited they are then lithified (turned into rock) through compaction and cementation. Chemical precipitation in pore spaces between grains 'glues' the rock together giving sandstone it's strength.

The Rock Cycle


Stone Colours
Different minerals form the different colours and compositions of different stones whether they be igneous, metamorphic or sedimentary. A list follows of the different minerals which predominantly make up the different stone colour.

Stone Colour Black Brown Grey Green Red White Yellow

Mineral Biotite, Hornblende Limonite Variety of minerals Mica, Chloride, Silicate Hematite Feldspar, Calcite, Dolomite Limonite

Hardness of Stone
The hardness of stone varies greatly from stone to stone. In the 1800's a scale was produced to measure and categorise the difference in hardness between stones. This scale is called the MOHS which is Measurement of Hardness Scale for stone. The hardest stone known to man is Diamond so that is rated at 10 and Talc is the softest so it is 1, all other stones have been scaled between these two benchmarks accordingly.

MOHS
>	Talc
>	Gypsum
>	Calcite (most marbles
>	Fluorite
>	Apatite
>	Feldspar (granite)
>	Quartz (granite)
>	Topaz
>	Corundum
>	Diamond

If a stone has a larger MOH it can scratch a stone with a lower MOH but a stone with a low MOH can never scratch a stone with a higher MOH. This must be  taken into consideration for paving in buildings, footpaths, etc, as the grit accumulated on the bottom of people shoes has a MOH factor of around 6 or 7, this means a granite with a MOH of around 7 is needed to prolong it's life.