Nevadite—a variety of rhyolite of granitoid aspect due to an abundance of porphyritic crystals.
Nodules—concretionary aggregations of rounded form.
Norite—a fine-grained rock consisting of plagioclase and hypersthene.
Novaculite (honestone, oilstone)—a very fine-grained, hard sandstone or silt-stone, used for whetstones.
Obsidian—a typical form of volcanic glass usually of the acidic class.
Onyx—a variety of chalcedonic quartz having colored bands alternating with white; the “Mexican onyx” is a crystalline calcium carbonate, variegated with delicate colors due to iron and manganese.
Oolite—a limestone or dolomite composed of small concretions resembling the roe of fish.
Ooze—an exceedingly soft watery deposit of the deep sea; characterized usually by microscopic shells from which it is mainly derived, as diatom ooze, globigerina ooze, etc.
Orthophyre (orthoclase porphyry)—a rock consisting of crystals of orthoclase in an aphanitic base.
Peastone (pisolite)—a very coarse variety of oolite.