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.
