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"A rock is an aggregate of mineral grains, fragments of
previously existing rock, or a mass of natural glass. Rocks can form
in a variety of ways—through freezing of a melt, by cementation of
loose grains, by precipitation from water solutions, or from changes
that happen in response to temperature and pressure underground.
Geologists can identify rocks and interpret aspects of Earth’s
history by studying two principal characteristics of rocks: rock
composition (the identity of minerals or glass that make up a rock)
and rock texture (the dimensions and shape of grains, and the ways
that grains are arranged, oriented, and held together)."
From: Laboratory Manual for Introductory Geology
by Ludman and Marshak