Geology Home Page
last edited: 12/22/15
What is a Crystal?
There are many definitions of what is a crystal. Most of the definitions
but others are drastically different.
The oldest definition is probably the simplest. Minerals were found that
surprisingly different than common rocks. These minerals were rare, had flat,
often shiny surfaces arranged in interesting geometrical patterns. 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.
Within a solid that possesses crystalline structure, the atoms are arranged in a
repeating geometrical pattern. A crystal is a piece of solid material (element or
chemical compound) in which the geometrical pattern is coherent throughout the
entire object. The external shape is of no significance because crystals often form
in crowded conditions and their outer shape is deformed.
geometrical arrangement of sodium and chlorine atoms in a salt molecular structure
geometrical arrangement of carbon atoms in the molecular structure of graphite
A third definition is tied to the external shape of crystals. If a small
(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. These flat surfaces, called faces, will reflect the symmetry of the
underlying layers of atoms.
Calcite and halite are good examples. Note how the shape of both calcite and
halite reflect their internal arrangement of atoms at the molecular level.
calcite molecular model showing rhombic arrangement of atoms
calcite crystals showing rhombic shape
halite molecular model showing cubic arrangement of atoms
halite crystals with cubic shapes
A fourth definition of the word crystal has nothing at all to do with real crystals
but stems from history. Before the story can be told, it is necessary to explain
what glass is. Glass is almost the exact opposite of the definition of crystal. In a
glass atoms are arranged at random with respect to each other; their are no nice
neat geometrical patterns. 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). The liquid is quickly cooled, not allowing the atoms enough
time to arrange themselves into geometrical patterns.
It is ironic, therefore, that a variety of glass with a high lead content is referred to
as "crystal". Many, many years ago in Europe, people found nice shiny crystals
of the mineral quartz.
They took the quartz crystals home, tied strings to them and hung them where
candle light could make them sparkle. At a later time, the tradition continued and
the crystals were hung from chandeliers with candles. When the supply of crystals
diminished, people took and cut pieces of glass to hang in place of the crystals.
They continued to call the cut glass crystals. 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. Today, all forms of glass bowls , vases, and
decanters made from this leaded glass are referred to as "crystal". This definition
of crystal does not apply to the scientific definition of crystal.