Freestone—a sandstone of uniform grain without special tendency to split in any direction.
Fulgurites—glassy tubes, produced through fusion by lightning in penetrating sand, earth, or rock.
Gabbro (euphotide)—a crystalline rock composed of the plagioclase feldspar, labradorite (or anorthite), and diallage (or a related ferromagnesian mineral), with magnetite or titaniferous iron as a common accessory.
Gangue—a term applied to the crystalline material in which ores are imbedded.
Gannister—essentially a quartz silt or pulverized quartz used for lining iron furnaces.
Garnetite—a rock composed largely of garnets.
Geest—residual earth or clay left by the decomposition of rocks, especially limestones.
Geyserite—the silicious sinter deposited about hot springs.
Globulites—minute spherical bodies embraced in volcanic glass.
Gneiss—a foliated granite, consisting typically of quartz, feldspar, and mica; the feldspar typically orthoclase.