Quartz occurs in granite as the lustrous, partially rounded grains which constitute perhaps 20% of the rock (feldspar makes up most of the more opaque remainder which breaks with many small flat faces), and is recognized in the small glistening grains in the porphyry. Hence it is an important igneous rock-forming mineral. Sandstone in Missouri is made almost entirely of quartz grains which have been broken, worn, and more or less rounded during their long travel history. The sandstone formation quarried and mined near Pacific and Crystal City, named the St. Peter sandstone formation, is an outstandingly pure quartz sandstone and therefore usable in the glass industry. It is obvious that quartz is an important constituent of sedimentary rocks. [Illustration: Lustrous, translucent quartz. The irregular fracture and oily luster are characteristic.] Further, in the sedimentary dolomite formation near Potosi, fine to coarse quartz crystals line the surfaces of cavities and pockets in the stone. This cavity coating of quartz which reflects light brilliantly from the many small crystal faces is called drusy quartz by the mineralogist, but is locally and popularly known as “blossom rock.” Thousands of pounds of “blossom rock” are sold each year for rock gardens and various ornamental purposes. In northeastern Missouri quartz crystals line the hollow, more or less spherical bodies called Geodes, which vary in size from small nuts to melons, and weather out of the so-called Warsaw formation. Other types of hollow cavities in many Missouri rocks may contain quartz growing inward from their walls. Missouri chert is composed primarily of quartz in microscopically fine grains; likewise, agate and petrified wood may contain abundant quartz. Other varieties are rock crystal, rose quartz, amethyst, false topaz, bloodstone, carnelian, and onyx. Quartz crystallized in an igneous rock as the hot fluid cooled through its “freezing” temperature interval, which was probably not below 1000°F. In the cases of the quartz in geodes, the drusy quartz, or that in cavities within petrified wood, quartz crystals grew from ground water solutions which must have carried very low concentrations of silica in solution, and whose temperatures did not depart far from that of rocks buried at various depths today. Although quartz is a very common and abundant mineral, our specific knowledge about its transport and deposition is surprisingly meager. [Illustration: Quartz crystal cluster. Crystals are six-sided. From Arkansas.] Quartz crystals are used in large quantities in radio apparatus where it is necessary to maintain very close control on the tuning of a circuit. This use requires quartz of highest quality and crystals above minimum size, which have never been found in Missouri and probably are not present. Silica production from this state is in its sandstone, tripoli, chert chats, and rock garden ornamental stone.