NATIONAL PARK SERVICE: Rocks & Minerals: https://www.nps.gov/subjects/geology/rocks-and-minerals.htm
SMITHSONIAN INSTITUTION: GeoGallery: https://geogallery.si.edu/gems-minerals-meteorites-rocks
Smithsonian Rock Gallery: https://geogallery.si.edu/rocks
Radford University, Virginia: https://www.radford.edu/jtso/GeologyofVirginia/Rocks/GeologyOfVARocks2.html
Oregon State University: Rock & Minerals: http://volcano.oregonstate.edu/book/export/html/196
Geological Survey Ireland (Go to this page first. Sub pages are: Rock types, Rock Cycle, How old are rocks?): https://www.gsi.ie/en-ie/education/rocks-minerals-and-gems/Pages/default.aspx
Missouri Dept. Of Natural Resources (20-page PDF that explains the 3 types of rocks, rock cycle, and minerals with an emphasis on Missouri natural resources): https://dnr.mo.gov/geology/docs/rocksetbooklet.pdf
Univ. of California San Diego: https://topex.ucsd.edu/es10/lectures/lecture10/lecture10.html
Univ. of Wisconsin: School Library Education Consortium: 3 types of rocks (11 min. video): https://uwsslec.libguides.com/c.php?g=214976&p=1578178