High temperature and pressure conditions within Earth’s crust provide for minerals to develop when molten, silicate-rich liquids (magma) harden, or crystallize, upon cooling. Depending on the composition of the magma and its cooling history, many interesting kinds of minerals are produced.
Minerals that formed in a magmatic environment are displayed in these two cases. A magma is a mass of molten material, predominantly silicate in composition, that exists usually between 600 and 1300 degrees centigrade, deep within the Earth. When the temperature of, and the pressure on, the magmatic body decreases, various kinds of minerals are able to form and produce a kind of rock material known as igneous rock. In the case on your left is shown an igneous rock called gabbro. And in the other case, an igneous rock, fairly commonly known, that is granite. Cyanite, another kind of igneous rock, also is displayed.
Usually the minerals are sparsely scattered throughout and consist of the ordinary common rock-forming minerals, such as feldspar and quartz, neither of which are shown here. Instead, minerals, such as chromite, plantiniferous pentlandite, and even diamond may develop in rather unusual concentrations that may be rich enough to warrant their extraction. As with minerals formed in a metamorphic environment, long term erosion, together with crustal uplift, must take place before such minerals become available to you and me for our direct observation.