Starting from the basics, crystals are
materials made of closely packed atoms or
molecules (the smallest unit of material) arranged
in a very specific 3-dimensional pattern. As the
pattern continues over large distance they form
the geometric shapes that we see (think of a
diamond, rock salt, or even a snowflake).
In general, crystal shapes are determined by
the energy of the atoms (or molecules) in the
material. Things always want to be in the lowest
energy arrangement. Depending on the size, shape,
and electronics of atoms or molecules, this leads
to different patterns. Once a crystal pattern
starts, it is almost always favorable for atoms
(or molecules) to continue the pattern and grow
into a crystal.
Crystals often need time to form these patterns,
so most crystals are formed slowly over time.
(Diamonds are formed over billions of years and
crystals in caves are often formed over hundreds
of thousands of years!) This gives the atoms or
molecules a long time to find their place in the
pattern (reach the lowest energy).
Sometimes there are multiple patterns that give
low energies, like in snowflakes, so there are
multiple crystal shapes for a single material.
Hope that helps!
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