Rockhounds and Rocks
Collectible Rocks and Minerals
Rocks contain clues to the geologic history of the earth and even the solar system. They tell of the earth’s violent and fiery origin some 4.6 billion years ago, pointing to a process of “density stratification,” when earth’s raw materials compelled by gravitational attraction sorted themselves according to their density, forming a solid inner core, a molten outer core, a very hot but still generally solid mantle and a thin crust. Rocks recall the formation of an atmosphere and the oceans. They show the effects of volcanic eruptions, drifting continents, fracturing and warping structures, changing climates, advancing and retreating glaciers, advancing and withdrawing oceans, flowing water, relentless winds, and meteorite impacts.
Moreover, rocks chronicle in a scrambled and fragmented way the capricious and episodic development of life on earth over 3.5 billion years. The fossil record documents the reign of single-cell microbes for three billion years, the emergence and florescence of multicellular life over the last 600 to 530 million years, and the ascendancy of invertebrates, marine vertebrates, reptiles, birds and mammals in the last half billion years. Even with chapters missing, the rocks still tell us, says Stephen Jay Gould in a Scientific American article, that “The history of life tends to move in quick and quirky episodes, rather than by gradual improvement.”
In a rainbow of colors, a treasure-trove of crystals and a graveyard of frozen life forms, rocks offer us a tantalizing look at our restive planet, our biological predecessors and even our majestic solar system. It’s enough to make a rockhound out of anyone.
How Are Rocks Formed?
Rocks in the crust of our planet form in three different ways. First, molten material, or magma, from well below the surface, rises through fractures and fissures to fill underground chambers, cooling and solidifying to become granite, or it may erupt from volcanoes to blanket the landscape, cooling and solidifying as lava or as light frothy pumice. Granite, lava and pumice bear the name “igneous” rocks. Second, stone fragments and sediments produced, transported and deposited by water, ice and wind, working in concert with gravity, coalesce and solidify over millennia to become “sedimentary” rocks. These include, for example, sandstone and limestone. Third, igneous or sedimentary rocks, subjected to pressure and heat induced by compaction, structural faulting or folding, or magmatic intrusions beneath the earth’s surface, may change into a new, chemically altered form called “metamorphic” rocks, including, for instance, slate, marble or anthracite coal. If any of the rocks reach high enough temperatures, they may melt, becoming molten, beginning anew the cycle of rock formation.
Igneous, sedimentary or metamorphic rocks, which comprise aggregates of different mineral grains, may also contain mineral concentrations, which are uncombined native elements such as gold or silver or are compounds such as mica or feldspar. The rocks’ mineral concentrations condense or crystallize from fluids that entrain elements and invade crustal fractures and voids.
Collectible Rocks and Minerals
In the desert Southwest, good rocks and minerals those stony aristocrats that you might, as a rockhound, like to collect and display on your mantle or make into jewelry include, as a few examples, geodes, turquoise, quartz, fluorite, malachite, volcanic bombs, obsidian, fossils and, from beyond the earth, meteorites.
Geodes, among the most prized rocks for collectors, have a roughly spherical shape that typically spans several inches in diameter, and they have hollow interiors that come lined with crystals of several minerals, usually quartz of various colors. The crystals may have formed, over millions of years, from minerals introduced by ground water infusing into cavities left by bubbles of gas once trapped within an igneous rock matrix. The color variations reflect differing constituents of the water. Geodes’ hard outer shells may have formed during the infusion of the water into the cavities. You will find geodes in many places across the Southwest, for instance, near Ludlow in southern California or at the Rock Hound State Park and Spring Canyon Recreation Area in southwestern New Mexico.
Turquoise, the iconic gemstone of the desert, typically occurs as veins or rock crusts in association with copper, which imparts a bluish color, or with iron, which imparts a greenish color. Usually opaque with a waxy luster, “Turquoise is the rare and improbable product of an incalculable number of chemical and physical processes that must take place in the right combination and proper environment over a time span of hundreds of thousands if not millions of years,” according to the Southwest Silver Gallery Internet site. Turquoise occurs at various sites across Nevada, Arizona and New Mexico. Especially high quality turquoise comes from the Dameli Mine in east-central Nevada. Some of the most famous comes from Cerrillos, between Santa Fe and Albuquerque, New Mexico, the earliest mining area in North America.
Quartz transparent or translucent, white, yellow, red, brown, black ranks among the most common of the minerals. It forms from the two most abundant crustal elements, oxygen and silicon. Found in veins or sheeted zones, often in association with metal ores, quartz occurs in igneous, sedimentary and metamorphic formations as a six-sided crystal with pyramidal-shaped tips. Its crystals may vary from a flyspeck up to a barrel in size. They form in fissures, from elements introduced by hot water. The amethyst variety usually holds the highest value as a gemstone. In the Southwestern desert, quartz occurs, for a single example, in massive outcrops in Quartz Peak, southwest of Phoenix. Click here to see some quartz products.
Cubic translucent to clear fluorite crystals, which occur in a wide range of colors, fluoresce vividly under ultraviolet light (hence the name). Fluorite, according to Patrick M. Colgan, Northeastern University, Rocks and Minerals Dictionary Internet site, may appear to have crystals within the crystals. “A fluorite crystal,” he says, “could have a clear outer zone allowing a cube of purple fluorite to be seen inside... One crystal of fluorite could potentially have four or five different color zones or bands.” Fluorite can form at relatively shallow depths in sedimentary rocks such as limestone, where strong brines with the prerequisite constituents may invade the fissures. It occurs, for a couple of examples, in the mountains around Wickenburg, northwest of Phoenix, or at the small peak called Bishop’s Cap, south of Las Cruces, in southern New Mexico.
Malachite, a forest green semi-precious gemstone marbled with pale green streaks, usually takes the form of a globular mass although it sometimes occurs as a spray of porcupine quill-like projections. Strikingly beautiful and easily workable, malachite has found a place in the craft of jewelry making since prehistoric times in some parts of the world. It forms when copper interacts with carbonated water or limestone. It occurs commonly in the copper mines of Arizona and southwestern New Mexico.
Volcanic bombs, which blast like cannonballs out of a crater vent during an eruption, sometimes form as a glob of magma encapsulating a cluster of olivine crystals, with a few rich enough in color and quality to be rated as the gemstones called peridots. The olivine crystals, translucent to transparent, range in color from green to yellowish green to white or brown. They have a high melting temperature. The best peridots, which have a bright green color, found places as settings in the jewelry of Egyptian royals. Volcanic bombs with olivine crystals and peridots have been found in Kilbourne Hole and one other volcanic crater in south-central New Mexico.
Fossils the mineralized remains or the impressions of plants or animals offer a whole new dimension to rock collecting, namely, a record of the emergence and development of life in changing environments on earth through hundreds of millions of years. Fossils almost always occur in sedimentary rocks. “The process of fossilization [of plant or animal remains],” says the Enchanted Learning Internet site, “involves the dissolving and replacement of the original minerals in the object with other minerals” Impressions of, for instance, plant leaves or animal tracks left in sediment, will be preserved if the sediment turns into rock before the impression erodes away. Fossils commonly occur in limestone deposits. Fossil treasure troves occur in every state in the Southwest.
Meteorites don’t speak to the history of the earth, of course, but they do tell us something of the early history of our solar system, bringing yet another dimension to collecting rocks. Broadly classified as stony, stony iron or iron, meteorites may represent debris left over from the formation of the solar system or material blasted into space by surface collisions on other solar system bodies, for instance, the moon or Mars. Typically, meteorites disintegrate as they heat up from friction when entering the earth’s atmospheres, 50 to 70 miles above the surface, and the fragments fall to earth in a scatter across what is called a “strewn field.” One meteorite strewn field lies in Arizona’s Mojave County, where the University of Arizona has worked to recover and analyze fragments and map the impact area.
Where Do I Look for Rocks and Minerals?
You will discover interesting rocks in numerous places across the desert Southwest, many of them mentioned in DesertUSA articles. You will find The Gem Trails Guides series of books to be especially useful guides to rewarding locations. For instance, the books provides text, maps and photographs for more than 80 proven sites in the Gem Trails of Southern California. It provides comparable information and directions in gem trail books on Nevada, Arizona, Utah, New Mexico and Texas. (You can acquire his books through DesertUSA’s Trading Post, Chris Pellant’s Rocks and Minerals, a descriptive reference with clearly written text and excellent photographs, will serve as a useful field guide for collectors.
What Equipment Do I Need?
In the field, you will need a geologist’s rock hammer, chisels, a good knife (for testing sample hardness), a hand lens (for examining crystals and grain), cloth and plastic sample containers, a topographical map, a compass, and a note pad (for documenting your finds). Importantly, you should come equipped with safety equipment, especially safety goggles, a hard hat and heavy work gloves. An injured eye, fractured skull or shredded hands can spoil your whole day. At home, you will need scrapers, brushes and, preferably, distilled water for cleaning samples.
How Do I Identify Rocks and Minerals?
Often, with a reference such as DK Handbook of Rocks and Minerals combines 600 vivid full-color photos with descriptions of more than 500 specimens. This authoritative and systematic photographic approach, with words never separated from pictures, marks a new generation of identific. 256 pages with index. You can identify a rock or mineral by a simple examination of physical characteristics such as color, hardness, luster, cleavage, crystalline form, specific gravity and magnetism. For instance, olivine, from volcanic bombs, has a color that is greenish, a hardness that is greater than a steel nail, a luster that is dark and nonmetallic, and cleavage that is jagged. For another rock or mineral, you may have to determine the density (weight for a given volume) or the magnetism to aid in the identification. You may have to submit others to chemical or fluorescent light tests.
Often, one of the most definitive characteristics of a rock or a mineral is its hardness, which you can determine by comparing it against the standardized Mohs Scale of Hardness:
MOHS SCALE OF HARDNESS
Your fingernail has a hardness of about 2.5, so it would leave a scratch on a gypsum stone. Glass has a hardness of about 6 to 7, so it would scratch the rocks and minerals with a hardness of 1 through 5. A copper penny has a hardness of about 3, a knife blade, of about 5.5. You can buy a test kit of hardness-testing tools with points calibrated to the Mohs scale.
Can I Collect Rocks Legally on Public Lands?
Typically, you may be allowed to take “reasonable” amounts of rocks, minerals, invertebrate fossils and petrified wood in non-restricted areas, without using power equipment, for your personal collection, but because regulations vary with the location and the administrative state or federal agency, you should check with the proper local officials before you begin collecting.
Break at Home Geodes
Meeting Other Rock Hounds
If you would like to get acquainted with other rock hounds, you will likely find an association, society or club nearby. Visit the United States Club List, produced and maintained by rockhound Albert Zabinski. Here you will find an “online list of over 900 U.S. Gem, Mineral, Lapidary, Fossil, and just about anything to do with rocks clubs, guilds, associations, and societies”
Rock Hound Mecca
Every January and February, hundreds of thousands of snow birding and RVing rock hounds make a pilgrimage to Arizona, especially to Quartzite and Tucson, to renew old friendships, gossip, compare notes, tell yarns, swap rocks and minerals, and buy and sell specimens. In Quartzsite, in western Arizona, they shop at eight major shows, where 2000 vendors sell rocks, minerals, gems, fossils and handcrafts in one of the world’s largest outdoor flea markets. In Tucson, at the annual gem and jewelry show the largest in the world they can visit nearly 40 different venues during late January and early February. They can, as the Whirled Planet Internet site says, “find everything from fossils and mineral specimens to beads to high end diamond jewelry. If it’s made from stone, chances are [they] can find it in Tucson.”
Rocks and Kids
Families often discover that scouring the countryside for rocks can be as much fun as panning streams for gold. It often yields more immediate rewards. Moreover, kids may discover that a rock can have a purpose almost as exciting and satisfying as serving as a missile for smashing a window or smacking a cousin’s head. In fact, youngsters may come back from a trip telling stories about the stories that rocks tell.
By Jay Sharp
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