Silicon dioxide, better known commercially as glass (and otherwise as quartz), is the second most common mineral on earth. Being one of the harder substances, it erodes more slowly than most other minerals, so smaller particles accumulate as sand. The macrocrystalline varieties of quartz have attractive and visible crystals, but microcrystalline varieties (with crystals that can’t be seen without magnification) are common. Forms of quartz include agate, onyx, jasper, Tiger’s eye, amethyst, citrine, chalcedony, chert, flint, and rock quartz.
Quartz (and glass), including the microcrystalline types, has conchoidal fracture. This means that the breakage pattern produces shell-like rings. Stone-age technology used controlled fracturing of stones such as chert and flint because they were easier to obtain and to work to produce points such as arrowheads. Arkansas is well known for the chert-like stone called novaculite that is common in part of the Ouachita Mountains.
Chert that has been worked into a stone point by controlled conchoidal fracturing.
Arkansas also is well known for the quartz belt that runs through the Ouachita Mountains. It is believed that the quartz found at the surface today was formed over 250 million years ago in fissures of rocks that were a mile or more below the surface of the earth. The most familiar quartz from the Ouachitas was formed when open fissures in the bedrock became filled by crystallization of quartz from hot, silicon-rich waters. Mountain building pushed the veins upward and broke some of the crystals in the process. On the other hand, veins of quartz helped hold highly fissured sandstone together. (Pictured: Milky quartz vein in sandstone)
Quartz forms in several rock types. Veins that run through sandstone often produce clear crystals, and veins in shale more often produce a variety called milky quartz. The milky appearance is due to numerous tiny spaces which refract light. (Common quartz crystal, left; smoky quartz crystal, right)
Without impurities, crystals may be "crystal clear." Some radioactivity during development caused the grayish form known as smoky quartz (note: rock shops sometimes irradiate natural quartz to produce this variety for sale).
The process of formation of some crystals trapped water in bubbles, creating "enhydros." An enhydro (with bubble outlined with box) is pictured. The bubble is magnified at right.
If quartz was broken during the growth process, it can "reheal," but pressures may also cause fractures within a crystal. When held in sunlight, a "rainbow effect" is produced in the area of a fracture. The nature of the colors depends on the thickness of the space in the fracture (Pictured left: clear quartz with rainbow fracture). Quartz also has interesting properties. A small amount of electricity applied to a crystal makes it vibrate (the piezoelectric property), which is used for quartz watches and similar applications. Also, when two pieces of quartz are rubbed against each other they will produce a flicker of light, called triboluminescence, and a metallic odor. Quartz has been associated with the presence of gold, and there were gold rushes and mines dug in the Ouachita Mountains of Arkansas, but no gold was found in the veins.
Druzy quartz refers to closely spaced, small crystals that form over the surface of other rock.
Agate is the name given to banded and often colorful forms of chalcedony.
Amethyst has a purple coloration due to included minerals. Most commercial amethyst available at rock shops comes from Brazil.
Citrine occurs naturally, but commercial specimens often are made by processing amethyst.
Tiger's Eye is a fibrous quartz that contains brown iron that produces its golden-yellow color.
Polished Tiger's Eye show chatoyancy, in which reflected light seems to move over the stone and resembles the eye of a cat.
The interior of geodes often are lined with druzy quartz.
Herkimer diamonds are a small form of water-clear quartz found near Herkimer, New York.
Chalcedony refers to any microcrystalline form of quartz, but is most commonly used for the lighter colored materials, such as this pink chalcedony.