Crystals of quartz are typically six-sided, elongated, have sharply pointed pyramid-like ends, and are apt to grow together forming twins. Good crystals are rare in Illinois, and the crystal structure is not apparent in the commonly occurring grains and masses. Quartz is brittle and hard. It may be colorless or tinted, transparent or translucent, but more commonly it is white and nearly opaque. Transparent quartz looks much like ordinary glass, but it scratches glass easily. It has a glassy to brilliant luster and breaks irregularly or with a good conchoidal fracture. Some varieties of quartz that are used for semiprecious gems are chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, generally translucent, white to gray, blue, brown, or black. Agate 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 quartz, generally is red. 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 and 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. FELDSPAR (22) FELDSPAR is the name applied to a group of minerals that are the second most common of all the earth’s minerals. All feldspars are composed of aluminum, silicon, and oxygen, combined with varying amounts of one or more metals, particularly potassium, sodium, calcium, and lithium. The minerals are hard, have a smooth glassy or pearly luster, and cleave along two planes nearly at right angles to each other. Feldspars are fairly light weight. The streak is white, but the color of the mineral is highly variable, although potassium and sodium-bearing feldspar are commonly white or pink and most plagioclase feldspar is gray. Feldspars are essential parts of the crystalline igneous rocks. Their decomposition products are present in most soils. In Illinois relatively small feldspar crystals can be found associated with quartz and other minerals in granite and gneiss boulders and pebbles in glacial drift. MICA (23) MICA is the name of a family of complex aluminum silicate minerals that can be split easily into paper-thin, flexible sheets. If broken across the grain at right angles to the flat, smooth surface they fracture raggedly. In a single mica crystal the sheets range from more or less transparent to translucent and are arranged one on top of another like a deck of cards.