Hexagonal pyramidal end of a single quartz crystal

Quartz (SiO2) is the most common of all minerals, making up 12% of the Earth's crust. There are two main types of quartz: (1) crystalline quartz and (2) dense, microcrystalline to cryptocrystalline (microscopic to submicroscopic) quartz. Many dense varieties occur in Illinois; the most common is chert.

Well-formed, prismatic crystals of quartz are typically six-sided and elongated with sharply pointed pyramid-like ends. Quartz crystals are apt to grow together, in clusters. Good, large crystals are rare in Illinois. However, good, well-formed crystals occur within some geodes (see Subsidiary Rock Forms) and within certain openings (vugs) in some limestone layers.

Cluster of quartz crystalsQuartz is brittle and hard. It may be colorless or tinted, transparent or translucent; more commonly, it is white and nearly opaque. Transparent quartz appears similar to ordinary glass, but scratches glass easily. Transparent quartz has a glassy (vitreous) to brilliant luster and breaks irregularly or with a good curved (conchoidal) fracture. Specific gravity is 2.65.

Some varieties of cryptocrystalline quartz that are used for semiprecious gems are chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, generally translucent, and white to gray, blue, brown, or black. Lake Superior agatesAgate is a form of chalcedony that has a mottled or variegated banded appearance and may be yellow, green, red, brown, blue, gray, or black. Onyx is agate with parallel bands that as a rule are brown and white or black and white. Jasper, an impure opaque chert, generally is red or yellow-brown.

Quartz occurs as rock crystal (colorless, transparent), milky quartz (white, nearly opaque), and smoky quartz (smoky yellow to gray or brown) in geodes from the Warsaw and Keokuk Limestones of the Nauvoo-Hamilton-Warsaw area. Quartz also occurs as vein and cavity fillings associated locally with fluorite, sphalerite, and galena in extreme southern Illinois. It also occurs as vug (cavity) fillings in limestones and sandstones.