Feldspar—a group of minerals of the first importance in rock formation, embracing orthoclase, microcline, albite, oligoclase, andesine, labradorite, anorthite, and numerous variations; aluminum silicates, with either potassium, sodium, or calcium or two or more of these; crystallizes in both the monoclinic and triclinic systems; possesses very distinct cleavage in two directions; H. 6–6.5; range in color from white through pale yellow, red, or green, and occasionally dark; triclinic feldspars frequently called plagioclase (see individual feldspars).
Fluorite—fluorspar; calcium fluoride; isometric, usually cubic; H. 4; luster vitreous, sometimes splendent; white, yellow, green, rose, crimson red, violet, sky-blue, and brown; yellow, greenish, and violet most common; occurs usually in veins or cavities in beautiful crystalline form.
Galenite—galena; lead sulphide; isometric, usually cubic; perfect cubic cleavage; luster metallic; lead-gray; a common ore of lead; occurs in veins and layers, also as linings of cavities.
Garnet—a complex silicate of varying composition, embracing aluminum, calcium, magnesium, chromium, iron, and manganese, but usually only two or three of these are present in abundance, and the varieties are characterized by the leading constituent; isometric, usually in dodecahedrons or trapezohedrons; H. 6.5–7.5; luster vitreous to resinous; commonly red or brown, sometimes yellow, white to blue, green or black; common in mica schist, gneiss, hornblende schist; also in granite, syenite, and metamorphosed limestone.
Geyserite—a concretionary deposit of silica in the opal condition; formed about geysers; white or grayish.
Glauconite—green-sand, a hydrous potassium-iron silicate usually impure, amorphous, or earthy; dull olive-green or blackish, yellowish, or grayish green; opaque, commonly occurs as grains or small aggregations.
Graphite—plumbago, black lead; a form of carbon, usually impure; rhombohedral, but rarely appearing as a crystal; more often as thin laminæ of greasy feel; yields a black adhesive powder; hence its common use for lead pencils; occurs in granite, gneiss, mica schist, crystalline limestone; sometimes results from alteration of coal by heat; occasionally occurs in basaltic rocks and meteorites.
Gypsum—a hydrous calcium sulphate; monoclinic; perfect cleavage into smooth polished plates; occurs in a variety of forms, including fibrous and granular; H. 1.5–2; luster pearly and shiny; white, sometimes gray, flesh-red, yellowish, and blue; impure varieties dark; crystallized varieties include selenite, satinspar, alabaster, etc.; easily recognized by its softness and want of effervescence with acids; occurs in beds; calcined and ground constitutes plaster of Paris.
Haüynite—a complex sodium-aluminum silicate and calcium sulphate; crystals dodecahedrons; luster vitreous or somewhat greasy; bright blue, sky-blue, or greenish blue, or green; occurs in certain igneous rocks, commonly associated with nephelite and leucite.
Hematite—ferric oxide, Fe2O3, iron-sesquioxide; rhombohedral, more commonly columnar, granular, botryoidal, or stalactitic; luster metallic, sometimes earthy; iron-black, dark steel-gray, red when earthy; gives red streak or powder; a leading iron ore, 70 percent. metallic iron when pure; the chief source of the red color of soils and rocks generally.