ROCKS!
There are three types of rocks – igneous, metamorphic, and
sedimentary. Within these classifications are many hundreds of
rock types. Igneous and metamorphic rocks are formed under
conditions of extreme heat and pressure.
Igneous rocks are formed when magma cools into solid
form. There are over 700 varieties of igneous rock have been
named, some crystals, some not. Igneous is derived from the
Latin word for 'fire', ignis.
Ninety percent of igneous rocks are minerals. Quartz is among
the most familiar and abundant of igneous rocks. Clays and
feldspar are other igneous rocks.
Quartz crystal
Metamorphic rocks are formed when a preexisting rock,
called a protolith, is under conditions of high heat and pressure.
The high heat and pressure cause the rock to change. The
protolith might be an igneous rock, a sedimentary rock, or
another metamorphic rock. Slate, marble, and quartzite are some
examples of metamorphic rocks.
Marble
Sedimentary rocks are less abundant than the other two
rock varieties, composing only 5% or so of the earth’s crust.
Sedimentary rocks are formed in one of three ways – when bits
of a larger rock chip off and settle to the ground, when the
remains of plants or animals build up in quantity, or when a
solution containing a mineral leaves deposits over time. They
are named for their origin – sediment.
Information courtesy of “wisegeek.com”