Geologists
classify rocks in three groups, according to the major Earth processes
that formed them. The three rock groups are igneous, sedimentary, and
metamorphic rocks. Anyone who wishes to collect rocks should become
familiar with the characteristics of these three rock groups. Knowing how
a geologist classifies rocks is important if you want to transform a
random group of rock specimens into a true collection.
Information provided by: http://interactive2.usgs.gov