http://www.windows2universe.org/earth/geology/min_intro.html
http://www.kidsloverocks.com/html/physical_properties_of_mineral.html
http://www.childrensmuseum.org/geomysteries/cube/mohs_scale_alt.html
https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/what-are-minerals-used-for/
http://www.zephyrus.co.uk/rocktypes.html
http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html
http://www.rockhounds.com/rockshop/rockkey/index.html
http://geology.com/rocks/sedimentary-rocks.shtml
http://geology.com/rocks/metamorphic-rocks.shtml
http://geology.com/rocks/igneous-rocks.shtml
http://www.myschoolhouse.com/courses/O/1/69.asp
http://www.myschoolhouse.com/courses/O/1/69.asp