In science, a crystal or crystalline solid, is a solid material whose atoms, molecules or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The process of crystal formation through crystal growth is called crystallization.
Gemstones are sometimes referred to as crystals although this term is most frequently used with reference to crystal healing. Although generally considered to be acceptable and quite accurate, it is important to be aware that not all gemstones are crystals and not all crystals are gemstones!
The word crystal originated from the ancient Greek word "krustallos" which had the same meaning, but according to the ancient understanding of the word which referred to anything congealed by freezing such as ice. It used to refer exclusively to Quartz or Rock Crystal and meant a solid material which forms by a regular repeated pattern of molecules connecting together. Whilst in some solids, the arrangements of the atoms and molecules can be random or different throughout the material, but in crystals, the atoms are repeated in exactly the same arrangement throughout.
Crystals are created by movement in the Earth's crust. As layers of crust float on the liquid rock mantle, collisions and pressure create fractures that allow superheated liquids and gases, filled with different elements, to move rapidly towards the surface. As these liquids and gases cool, they often crystallize. The temperature, pressure, surrounding rocks and mix of elements all help to determine which minerals crystallize. Rocks which form in this way are known as igneous rocks.
When exposed to natural elements such as frost, wind and water, rocks and crystals begin to erode. Small particles are then washed into the sea where they form silt deposits which after many millions of years of extreme pressure, become compressed forming sedimentary rocks. Minerals of this type tend to be softer as they form at lower temperatures and pressures.
The third method of crystal formation is known as metamorphic. This is where igneous or sedimentary minerals have been re-subjected to heat or pressure in the Earth's crust. Although the particles do not melt, they may re-combine to form different crystals. As the Earth's crust is continually moving, these crystallization processes are always in progress and we only get to see the results when veins or beds of crystals are exposed by erosion or are lifted close enough to the Earth's surface that they can be mined.
Further reading:
Crystals
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