Rocks are usually grouped into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks.
U.S. told Russia that Crocus City Hall was possible target of attack (washingtonpost.com) More than two weeks before terrorists staged a bloody attack in the suburbs of Moscow, the U.S. government told Russian officials that Crocus City Hall, a popular concert venue, was a potential target, according to U.S. officials familiar with the matter.