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Granite (from Latin granum, meaning "grain") is a common and widely occurring type of intrusive, felsic, igneous rock. It has a medium to coarse texture, occasionally with some individual crystals larger than the ground mass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.
Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone.
Granite is a normal, geological, source of radiation in the natural environment. Granite contains around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.
Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites.
In buildings constructed primarily from natural granite, it is possible to be exposed to approximately 200 mrems per year.
Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils formed over or from particularly uraniferous granites can become a trap for radon gas, which is heavier than air.
However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not often an acute health threat or significant risk factor. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.
The Highland Mountains on Reach were mountains of granite, and the planet Onyx also had numerous deposits of granite forming hills and mesas. Harvest's parliament building was constructed of granite. Forerunner structures on Alpha Halo were also made using granite, with an intricate level of symbols embedded in their surfaces.