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To understand what rocks are, we first need  to  be  aware  that  a  mineral  is  an  inorganic  compound  that  features  an  orderly  internal  structure and is of a constant crystal form and  chemical composition.

All rocks  fall  into  one  of  three  categories:  igneous rocks are formed from molten magma  or lava from volcanoes; sedimentary rocks are  those which are formed as a result of sedimentary  deposits being compacted under pressure over  a  sustained  period  of  time;  and  metamorphic  rocks are those that change their structure (hence  “morph”) through pressure or heat - or in most  cases a combination of both.

When the Earth was originally formed, 100%  of its surface was igneous rock. Over billions of  years, due to the movement of tectonic plates,  changes  in  the  environment  and  weathering,  much of the world’s surface has been replaced  with  sedimentary  and  metamorphic  rocks.  Today, less than a third of the Earth’s surface  remains as igneous rock.

To avoid confusion, it is important to understand  that  there  is  a  big  difference  between  where  gems are formed and where they are later found.  For  example,  many  gems  are  discovered  in sedimentary alluvial deposits; however, virtually  no  gems  are  actually  formed  in  sedimentary  rocks.

Igneous rocks yield such gemstones as Apatite,  Beryl,  Topaz  and  Tourmaline;  Sapphires,  Emeralds, Spinel and many Garnets are created  via metamorphism.

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