Rocks & Other Mineral Sources

Major Rock Types



A Rock, by a simple definition, is a solid with more than one component of a mineral or mineraloid. A single crystal is not a rock; but two crystals that are joined together, even if they are the same mineral, are technically a rock. The minerals or mineraloids may be large enough to be easily identified (such as in a pegmatitic granite), barely distinctive grains (as in a schist), or in a mixture of microscopic grains such as in a slate. A rock does not even need to have crystals but may be in the form of a non-crystalline solid state or glass, an amorphous mixture in which the chemicals are not crystallized into minerals, such as in obsidian. Generally rocks are considered to only be natural objects, but sometimes man-made substances are included as rocks.

There are three main classes of Rocks. They are classified according to how they originated. Igneous rocks form from cooling bodies of magma. Over time, various weathering processes erode these rocks and the resulting particles or chemicals settle into beds and are compressed and cemented into sedimentary rocks. If these rocks are buried, heated and highly compressed they will be made into metamorphic rocks. If these rocks continue to be heated and compressed to the point that they melt, then the molten rock might eventually form another igneous rock.  This is called the rock cycle.  It forms a complete circle as one rock can be turned into another.  They can even form different rocks of their own class.  A sedimentary rock such as a sandstone can be weathered and eroded and those fragments might eventually end up as part of a shale, a different sedimentary rock.

There are well-known rocks on Earth that don't fit into these classifications: primordial rocks, represented by meteorites. These condensed in the vacuum of space from the same solar nebula as the Sun and planets.

Understanding rocks, their origins and classifications, is important to understanding the minerals that are found in them, found on them or are a part of them. Below are the major types of rocks:

IGNEOUS - Rocks formed from melted rock or magma

SEDIMENTARY - Rocks formed from the consolidation of sediments


METAMORPHIC - Rocks formed from heat and pressure


In addition to the hard things called "rocks", minerals are sometimes found in loose collections of material called "unconsolidated sediments". Here are some major categories of these sediments, plus a couple of special interest to mineral collectors:


Before the Earth was formed, before igneous or sedimentary or metamorphic was even possible, there were rocks and minerals. In the early solar system they condensed directly from the solar nebula, and are visible today as asteroids and comets in space, and as meteorites on Earth. :

See Steve's video interview about asteroid capture at Moonandback:
Part 1
Part 2
Part 3

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