Crystals are interesting

The scientific study of crystals and crystal formation is known as crystallography. 
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The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. 
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The word crystal is derived from the Ancient Greek word κρύσταλλος (krustallos), meaning both “ice” and “rock crystal”, from κρύος (kruos), "icy cold, frost"
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Common crystals include snowflakes, diamonds, and table salt; however, most common inorganic solids are polycrystals. 
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Crystals are often symmetrically intergrown to form crystal twins.
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But what really is a Crystal?
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There are many definitions of what a crystal is.  
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Most of the definitions overlap, but others are drastically different.
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The oldest definition is probably the simplest.  
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Minerals were found that looked surprisingly different to common rocks.  
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These minerals were rare, had flat, often shiny surfaces arranged in interesting geometrical patterns.  
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In addition these specimens were quite colorful compared to rocks and were often transparent.

   
A more advanced definition stems from the definition of crystalline structure.  
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Within a solid that possesses crystalline structure, the atoms are arranged in a 
repeating geometrical pattern.  
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A crystal is a piece of solid material (element or chemical compound) in which the geometrical pattern is coherent throughout the entire object.  
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The external shape is of no significance because crystals often form in crowded conditions and their outer shape is deformed.

   

 A third definition is tied to the external shape of crystals.  
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If a small object (mineral, chemical or element) with a coherent repeating geometrical pattern of 
atoms is allowed to grow larger from a melt, solution or cooling gas in an environment in which there are no physical barriers to its growth, layers of atoms will selectively be added to the structure in such a way that the object will develop flat surfaces.  
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These flat surfaces, called faces, will reflect the symmetry of the underlying layers of atoms.
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Calcite and halite are good examples.  

The shape of both calcite and halite reflect their internal arrangement of atoms at the molecular level.
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A fourth definition of the word crystal has nothing at all to do with real crystals but stems from history.  
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Before the story can be told, it is necessary to explain what glass is.  
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Glass is almost the exact opposite of the definition of crystal.  
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In a glass atoms are arranged at random with respect to each other; their are no nice 
neat geometrical patterns.  
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Glass starts out as a melt in which the atoms are moving around and not attached to each other because of the tremendous amount of thermal energy (heat).  
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The liquid is quickly cooled, not allowing the atoms enough time to arrange themselves into geometrical patterns.
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It is ironic, therefore, that a variety of glass with a high lead content is referred to 
as "crystal".  
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Many, many years ago in Europe, people found nice shiny crystals of the mineral quartz.
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They took the quartz crystals home, tied strings to them and hung them where 
candle light could make them sparkle.  
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At a later time, the tradition continued and the crystals were hung from chandeliers with candles. 
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When the supply of crystals diminished, people took and cut pieces of glass to hang in place of the crystals.  
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They continued to call the cut glass crystals.  
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Eventually it was discovered that if a large quantity of lead oxide was added to the glass melt, a type of glass could be created that would produce strong rainbows of color when cut and polished and then hung below chandeliers.  
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Today, all forms of glass bowls , vases, and decanters made from this leaded glass are referred to as "crystal".  
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This definition of crystal does not apply to the scientific definition of crystal.
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Roger Weller