Crystals are a solid material whose constituents are arranged in
a highly ordered microscopic structure, forming a crystal lattice that extends
in all directions. The process of crystal formation is called crystallization.
The word crystal is obtained from the Ancient Greek word κρύσταλλος which
means both ice and rock crystal. Examples of large crystals include snowflakes,
diamonds and table salt. Most inorganic solids are not crystals but
polycrystals which include most metals, rocks, ceramics and ice. Different
crystals have different meanings from metaphysics.
The largest concentrations of crystals in the Earth are part of
its solid bedrock. Some crystals are formed by magmatic and
metamorphic processes creating large masses of crystalline rock. The metamorphic
rocks are recrystallized which means they were originally fragmented
rocks like limestone but high temperatures and pressure have acted on them by
erasing their original structures and inducing recrystallization in the solid
state.
Water-based ice in the form of snow, sea ice and glaciers is a
very common display of crystalline matter on Earth. A single snowflake is
typically a single crystal, while an ice cube is a poly-crystal.
https://en.wikipedia.org/wiki/Crystal
Rocks are naturally occurring solid aggregates of one or more
minerals. The Earth's outer solid layer is made of rock. Rocks
have been used by mankind throughout history. From the Stone Age, rocks have
been used for tools. The minerals and metals found in rocks have been essential
to human civilization. The major groups of rocks are
igneous, sedimentary and metamorphic.
https://en.wikipedia.org/wiki/Rock_(geology)
Shells are hard, protective outer layers created by an animal
that tends to live in the sea. The shell is part of the body of the animal and
the empty seashells that are often found on beaches are because the animal is
dead. The term seashell usually refers to the exoskeleton of an
invertebrate. Most shells that are found on beaches are the shells
of marine molluscs, partly because many of these shells endure better
than other seashells. Other shells that can be found on beaches are those from
barnacles, horseshoe crabs and Brachiopods. Seashells have
been used by humans for many different purposes throughout history and pre-history.
https://en.wikipedia.org/wiki/Seashell
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