Educated as a geologist/geophysicist, I dealt with geophysics and hydrogeology on projects ranging from mathematical modeling in the laboratory to hand-on application in the field. On the job, along hiking trails, and in rural housing subdivisions in southern California, I have picked up a variety of rocks, some of which carry fascinating geological stories. I thought about doing something educational with my collection but never got around to it. Now, well into retirement, my promise is long overdue. This book is dedicated to my grandchildren, who will soon be ready to read it; and to my wife, Zora, who has shared joy and pain with me along my career path. The book title, Wandering in Rock Country, captures the essence of an opportunistic rock collection. I try to write one story for each rock specimen.
All rock images were photographed from my collections, supplemented with a few field pictures. Specimens have not been altered or enhanced except for some with obvious saw-cut faces or minor smoothing of rough edges on rare occasions. Most specimens were essentially picked from free-standing, loose pieces of rocks in southern California. Source locations, however, represent my best recollection, not based on written records. Also, as my writing proceeds, I add stories on new specimens that were purchased or gifted to me.
This book, not a product of systematic study, is a show-and-tell presentation, based mainly on observations of hand specimens. Some questions raised here could be resolved by instrumental analyses, but I have preferred to restrict my interpretations to what can be seen with the unaided eye. After all, observation of a rock’s beauty is all we can do in the field. Beyond beauty, however, does a particular rock have an interesting story to be told?
I have visited many museums over the years. Most exhibit rare, eye-catching mineral specimens. Although I appreciate the beauty and rarity, most of us have little chance of seeing those fascinating specimens in the field. I would like to know more about specimens than the information generally given in display name tags. What is the geological story behind each specimen? It will be beneficial if the display narrative can be more informative to the visitors.
For better or worse, I attempt here to set an example. I stretch my imagination and try to make up a short story for each stone. The book is divided into five chapters (categories) and the stories are sequenced in orders of figure numbering. The rock identifications are given in general terms with minimal usage of scientific jargon for more refined name classification. Some pieces might be misidentified or misinterpreted because natural staining, varnishing, or patination could have masked their true identities.
All stories are independent of one another; hence the readers can flip to any figure without losing continuity in context. In preparation of the draft manuscript, each story (text and picture) was mostly limited to one paper page (8 ½ x 11 inches) but as needed I let it flow over sometimes. The final format in print or blog may be inadvertently altered. Also, to avoid repetition, some pictures are presented without stories. I do not have an exotic mineral or rock collection, but I hope my story-telling about commonly available rocks is educational or entertaining to some rock hobbyists and enthusiasts as well as aspiring geologists.
The italicized text, if any, gives my philosophical or personal reflection about a stone’s story. Skip those distractions if you are interested only in the story of the rock.
Originally I had planned to give an imaginative nickname as a flavor to each stone. I tried this idea by asking for suggestive names from pupils at a primary school. I was very impressed by and pleased with their enthusiastic responses, but I was overwhelmed by the variety of imaginative names offered for every slide shown. Imagination is in the eyes of beholders, indeed! So, let us fantasize the stones in our own preferable ways.
Not intended for an academic endeavor, this book does not cite any reference, nor provides an index. It is written for personal collection with some daring, original thought. For relevant or more information, however, search for any keywords on the internet. My claimed observations and interpretations could be provocative, contentious, or even outrageous to some readers. Now, please let us pause for a moment! Few serious geologists would tell a story based on one piece of rock specimen alone. My version of each story is just a beginning for the complete narrative to be told. One must visit outcrops, observe rock samples in the context of their surroundings, make laboratory analyses, and synthesize data by modeling to generate a convincing geological model or story. Obviously I did not do what I have recommended. But let us see what we can say about each individual piece of rock specimen.
I thank Dr. John G. Bolm, one of the first Americans I met 50 years ago, for comments on parts of a very early version of the draft manuscript. He did not necessarily agree with my viewpoints and I have changed my own opinions a few times, especially after I have access through Orange Belt Mineral Society (OBMS) of San Bernardino, California, to cut some rocks for better visual identification. Dr. Bolm could not wait for my never-ending addition to and revision of the text. To my sorrow, he passed away in early 2018. Now is the time to wrap up.
It has been years since I began to write something about my collection and make the display stands for the specimens. Please spare me for flip-flop along the way in my writing styles. I have also pondered how to dispose my collection before I am gone. I hereby relinquish my mental burden of collecting rock specimens over the years by releasing this unorthodox book. Thanks for browsing through it and please comment on my mistakes by informing me at email@example.com.
A Crude Guide to Identify Rocks in the Field
A rock is named for its occurrence, texture, and mineral composition, not its chemical composition. If you stand in the field, you see sedimentary, igneous, or metamorphic rocks. If you have a rock specimen in hand, try to determine what type it is and what it may be called.
Is the specimen fossil-bearing? An affirmative answer means you have a sedimentary rock; otherwise it still can be anything. Is it layered? Smooth layer interface implies sedimentary origin but intertwining one (‘dog teeth’ biting into each another) is metamorphic; the latter also displays shining, silky luster and can be magnetic. Are the grains visible? If the grains are fairly uniform in size and type and are isolated by matrix of finer particles (concrete-cement like), the specimen is sedimentary; if the grains (crystals) interlock one another, it is igneous. Grains in metamorphic rocks tend to be isolated and have rough contact with their individual surroundings.
If the grains are not visible, it can be siltstone or shale and may not be visibly distinguishable from metamorphic slate; well the latter tends to be splintering, denser and pliable for making the so-called slate tiles. If the specimen is jumbled with grains of various shape, size, and type, you are holding a piece of landslide debris or products of other near-surface chaos or disasters.
Igneous rocks are light (felsic), intermediate, or dark (mafic) in color. If the crystals are visible, they are respectively granite, diorite, and gabbro. Those are plutonic rocks, meaning the magma has solidified at depths for the crystals to slowly grow and interlock with one and another. If the magma (lava) extrudes to the ground surface, rapid cooling produces much finer grained volcanic rocks: rhyolite, andesite, and basalt, equivalently and respectively. Igneous rocks are classified according to artificially agreed relative compositions of rock forming minerals: light-colored quartz, K-feldspar, Na-Ca-plagioclase, and dark-colored mica, amphibole, and pyroxene. Defying our wish, nature has its own way of occurrence and we modify the names accordingly. For example, granodiorite for diorite with appreciable contents of K-feldspar and quartz, and tonalite for ‘granite’ that is short of alkaline feldspar (orthoclase). Sometimes an igneous rock may bear visible crystals (phenocrysts) imbedded in crytocrystalline groundmass or glass, it is porphyritic and the rock is so named with adjective for a dominant feature. Occasionally we encounter ultramafic rocks such as pyroxenite and olivine-rich peridotite that have been exposed through deep crustal erosion or catapulted by volcanic eruption to the ground surface as mantle xenoliths. And of course, there are volcanic lava (basalt) and glass (obsidian); and for explosive volcanoes, there are chaotic jumble of pyroclastics, including bomb, pumice, and ash.
Common metamorphic rocks are, in progression from low to high grades: slate, phyllite, schist, and gneiss. Schistosity begins at phyllite and matures in schist, both having shining micaceous luster. Gneiss is characterized by alternating dark and white bands. At high grade, metamorphic rocks can be granular as granulite or hornfel. If partial melting has occurred, gneiss turns into migmatite with wiggling or contorted stripes. Marble is the metamorphic phase of carbonates and it can appear in various familiar forms as commercialized construction or decoration products.
At the end of the day, leave the professionals to handle rock identification while we appreciate the beauty of rocks and beyond.
Agate – A translucent variety of quartz (chalcedony) usually curved with alternating color bands. It occurs typically in the cavities of other rocks.
Amethyst – A translucent to transparent variety of quartz with purple color.
Amorphous – Material or mineral without crystalline structure is said to be amorphous.
Amphibole — A group of dark rock-forming minerals which are mainly hydrous ferromagnesium silicate, including hornblende, nephrite (soft jade). Amphibolite is a rock consisting mainly of amphibole.
Andesite – A reddish brown, fine grained volcanic rock, of which the feldspar is primarily of andesine composition (sodium-calcium silicate). Name originates from the Andes Mountains in South America.
Anhedral – As opposite to euhedral, an anhedral mineral means its crystal faces are not well developed in igneous rock.
Aplite – Fine-grained granite occurs in a dike that cuts across other rocks.
Barite – It is a dense crystal of barium sulfate (BaSO4).
Basalt – Dark, fine-grained, extrusive (volcanic) igneous rock, sometimes with columnar structure. It hears high content of calcium plagioclase but with little or no quartz.
Bedding – It is synonymous with ‘layering’ in sedimentary rocks.
Bicarbonate – A chemical compound with bicarbonic anion HCO3, e.g., calcium bicarbonate Ca(HCO3)2, or sodium bicarbonate NaHCO3.
Biotite – A black, dark brown or green variety of mica with sheeting or leafy texture.
Boulder – It denotes an isolated piece of rock with diameter greater than 256 mm or 10 inches.
Brachiopod – An invertebrate, lamp-shell like fossil.
Brecciation – Making breccia, i.e., forming angular fragments of rock.
Brittle – Hard but liable to break or shatter easily at small deformation (less than 3 to 5%). It is generally used in contrast to ‘ductile’ material property.
Calcite – A common rock-forming mineral, calcium carbonate. It is a principal constituent in limestone, marble, stalagmite, and stalactite. It can be a minor component in other rocks. It also can be a cementing agent for sediments.
Cap rock – As loosely used here, it means a residual rock layer that overlies other, like a cap.
Carbonate – As a mineral, it designates a compound with anion CO3, e.g., calcite, dolomite. As a rock, it means deposit of organic or inorganic debris from solution.
Carbonic acid – A weak acid, H2CO3.
Cataclastic — ‘Cata’ connotes a rock formed during catastrophe and ‘clastic’ means mechanical origin. Clastic sediments are assembled through transport by water, wind etc. But cataclastic quartz is formed in situ, not assembled through transport processes.
Cataclastic rock – A rock composed of angular fragments that are formed by tectonic crushing or fracturing. As used here, it also refers to vein quartz shattered by cooling contraction or pressure reduction through removal of overburden.
Chalcedony – A cryptocrystalline variety of silica (intertwined quartz and moganite). It is translucent with wax-like luster, sometimes fibrous. It occurs as aqueous or hydrothermal deposits. Chalcedony is a catch-all term, including multi-colored or curve-banded agate, opal (hydrated), onyx (parallel banded), jasper (reddish), and sedimentary chert, plus other varieties.
Chert – A hard, dense, dull to semi-vitreous sedimentary rock, consisting mainly of fibrous silica (chalcedony). It comes with a variety of color, mainly dark grey or brown. It has splintery or conchoidal fractures (also known as flint if occurred within limestone).
Chlorite – A group of platy, greenish minerals consisting of hydrous alumino-silicate with iron and magnesium.
Clastic sediment – Sediment transported to the depositional site by mechanical means, e.g., water, wind.
Clay – Any rock fragment that is less than 1/256 mm in diameter. It may also mean clay minerals with platy crystalline structure.
Cleavage – The breakage of a mineral along its crystallographic plane, for example, rhombohedral cleavage of calcite.
Conchoidal fracture – A shell-like fracture commonly appears in obsidian, quartz, and flint.
Concretion – A lump of material or mineral congregation that is physically distinguishable from its host rock. It is often misused as being synonymous to nodule. Concretion connotes continuous growth externally with additional source material. In this sense, deep-sea manganese nodule should be call manganese concretion.
Conglomerate – A coarse-grained, clastic sedimentary rock. Typically it has large grained granule, pebble, coble, or boulder embedded in fine grained matrix of sand, silt, or clay. Essentially it has grain size greater than sandstone. Its unconsolidated equivalent is gravel.
Continental shelf – It is part of the continental margin that lies between coastline and continental slope (which deepens toward much deeper ocean floor). Generally the continental shelf has a slope of 0.1 degree and at water depth of less than 200 m.
Core – As used here, the core has three different meanings: 1) It is the innermost part of the earth at depth greater than 2,900 km, consisting of a liquid outer core and solid inner core. 2) It means drill core, cylindrical samples collected from drill holes. 3) It is a piece of rock that serves as a nucleation seeding for external chemical deposition from solution.
Correlation – As used in stratigraphy, the determination of contemporaries of geologic formations or units.
Cross bedding – An internal pattern of bedding within a formation, the newer sediments are deposited at an incline angle to a previous depositional surface, in response to change of direction of current or wind.
Crust – The outermost layer of the solid earth. Its thickness ranges from a few kilometers under the ocean floor to a few tens of kilometers under the continent. Its lower boundary, called Moho, is the discontinuity in the speed of seismic waves as a function of depth. It also has the ordinary meaning of crust, a hardened, enclosing shell.
Cryptocrystalline – It is used to describe crystals that are too small to be identifiable with ordinary microscope but its crystalline structures are still identifiable with electronic microscope.
Crystal – A solid with repetitive, regular arrangement of atoms to exhibit natural, external faces of definitive form. ‘Glass crystal’ is not crystal by this definition for lacking crystalline structure.
Diapirism – The process by which the overlying rocks or formations have been ruptured or pierced by the underlying, low density layers such as salt bed that rises to form a salt dome. The piercing can also be mobilized by pressure change or heating to force the unconsolidated sediments to squeeze upward. Seismic vibration can induce liquefaction and piercing.
Differential erosion – A phenomenon resulted from difference in resistance to erosion.
Diffusion – Migration of elements or ions from areas of high to low concentrations. The diffusion can also be carried out by fluid transport in the mode of advective diffusion.
Dike – As used for igneous rocks, a tabular igneous intrusion that cuts or fills across the planar structure of surrounding rocks.
Diorite – A plutonic rock with composition intermediate between acidic (i.e., high silica content, like granite) and basic (gabbro, high magnesium and calcium content). It is characterized by intermixing of dark and light colored minerals.
Dolomite – A carbonate mineral with equal number of calcium and magnesium atom in its molecule. It looks like calcite but does not effervesce in weak acid test. The name may also refer to carbonate rocks dominated by dolomite.
Drag fold – A set of minor folds on the order of a few centimeters to meters, formed in the softer rocks between two competent rock bodies that move relative to each other. It is an indicator of the direction for relative rock displacement.
Dripstone – Usually it refers to carbonate rock formed through dripping of water and its evaporation in a cave, like stalactite (as hanged from cave roof) and stalagmite (as protruded from the cave floor). The usage is extended here to mean rock at the ground surface formed by water drops through evaporation.
Druse – A small cavity in which small, protruding crystals may grow, as druse quartz.
Ductile – As compared to brittle, it is able to sustain 5 to 10% deformation before the rock is fractured. It is a property that allows solid-state flow.
Enclave – An inclusion.
Exfoliation – The process by which thin shells of rocks are peeled off, like the peeling of onion skin.
Exhumation – The exposure of buried features or rocks by erosion.
Fault – A rock fracture that has relative displacement across it, on the order of a few centimeters to hundreds of kilometers.
Flash flood – A sudden flood caused by heavy rain of short duration. Its huge flow volume or rate can overflow channels, which are normally dry or of low flow rate.
Flint – It is synonymous with chert but a more restrictive usage for dark, brown variety in association with limestone.
Fracture – It is a break in rock due to mechanical failure under stress. It may or may not have displacement, including crack, joint, and fault.
Frictional heating – Heating originated from friction during displacement along a fault.
Geode – A hollow or filled spherical or sub-spherical cavity in rock. Typically it is walled by chalcedony with its cavity filled or partially filled with quartz or calcite crystals.
Graded bedding – A sequence of bedding of which an individual bed starts with coarse grain at the base and gradually reduces its grain size to the top; the cycle is typically repeated to have a sequence of graded bedding. A reverse grading from fine to coarse grains can happen too.
Granite – A light-colored, coarse-grained plutonic igneous rock (formed at depths) of which the quartz content ranges from 20 to 60 percent of the light components and the ratio of alkaline (sodium or potassium) feldspar to the total feldspar ranges between 35 and 90 percent.
Granodiorite – A coarse-grained plutonic rock with composition between granite and diorite.
Gravel – An aggregate of unconsolidated, round rock fragments with grain diameter greater than 2 mm, including different proportions of granules, pebbles, cobbles, and boulders. It consolidates to become conglomerate.
Gypsum – A hydrous calcium sulfate (CaSO4.2H2O). It is an evaporite and can appear in various crystal forms, for example, selenite.
Hiatus – A break or interruption in the continuity of sedimentation caused either by no deposition or by erosion removals before newer sediments were deposited.
Igneous – Related to rock or mineral that solidifies from magma (molten rock).
Inclusion – A fragment of rock that was formed earlier and included in the newer igneous rock. It is also known as xenolith or enclave.
Intermittent – A spring or a stream that discharges only during certain period of time but it is dry most of the time.
Jasper – It is an opaque chalcedony — cryptocrystalline silica formed with volcanic rocks, like lava. It is reddish brown.
Joint – A potential or actual fracture in minerals or rocks without displacement across it.
Karst – A type of topography that develops by dissolution of limestone, dolomite, or gypsum. It is characterized with sink holes, caves, depressions, stalactite, stalagmite, and subsurface streams.
Lamina – Very fine, thin layering in sediments or sedimentary rocks.
Lava – Magma (molten rock) that oozes to the Earth surface. It also refers to rock that has solidified from lava flow, such as rhyolite, andesite, and basalt.
Liesegang rings (banding) – Secondary, color rings or curved bands which precipitate from fluids that enter porous host rocks, notable in sandstone, lithified volcanic ash, and weathered granite.
Lichen – Symbiotic assembly of algae and fungi on rock surface in dry climate. The algae extract moisture and nutrients through photosynthesis. Algae supply food to fungi and in return, fungi provide protection to algae. Depending on species variety, lichen can exhibit various colors and forms.
Limestone – A sedimentary rock that consists mainly of calcite. It represents aggregate of organic debris or inorganic deposition from calcium-carbonate bearing solution.
Liquefied – It refers to the state of transformation of weak, incoherent, unconsolidated layers into liquid form that can pierce overlying layers. It is induced by seismic shaking or excessive overburden pressure.
Lithification – The process of transforming unconsolidated sediments into cemented and consolidated sedimentary rocks.
Lithostatic pressure – The pressure exerted by a column or stack of rocks, analogous to hydrostatic pressure by a column of water.
Magma – Molten rocks.
Mantle – The layer or shell between the crust and outer core of the earth. It is about 2,900 km thick.
Marble – Recrystallized limestone through metamorphism without going through melting.
Matrix – The fine-grained material that fills the interstices of coarse-grained crystals in igneous rocks or coarse-grained minerals or rock fragments; or a fine grained component of conglomerate.
Metamorphism – Transformation of existing minerals, rocks, or structures into assemblage of rocks in new temperature, pressure, and chemical conditions at depths far below surficial weathering and cementation zones. It is a solid to solid transformation.
Mica – A group of silicates with complex chemical formula and sheet-like structure. It can be readily split into fine laminae with luster on the surface. It can be transparent or translucent tinted with white (muscovite), black (biotite), golden (phlogopite) or pink (lepidolite).
Migmatite — A rock mixture of igneous (granite) and metamorphic (gneiss) rocks. It represents partial melting of metamorphic rock and recrystallization of the melt into igneous rock.
Monzonite – A rock with composition between syenite and diorite, with nearly equal amount of orthoclase and plagioclase but with little or no quartz.
Mottle – A spot, streak, or patch of dark color on the surface of rocks.
Mudstone – Rock equivalent to shale but lacking the fine lamination or fissuring.
Mylonite – Rocks formed through extreme brecciation and pulverization under intense mechanical force, such as happening during faulting displacement. The less competent components may show ductile flow that engulfs more competent fragments. Mylonite is named for texture and it can have various mineral compositions.
Nodule – A rounded or irregular shaped body that forms and embeds in sedimentary rocks. It is usually an aggregate of mineral grains and is harder than its surrounding rocks. Nodule stops growing after segregation of ‘alien’ or minor constituents from the host or major component is completed. See also concretion.
Oblate – A spheroid of which one axis is shorter than the other two of equal length.
Onlapping – An overlap of sedimentary layers with the overlying layer extending or pinching out more than the underlain layer.
Obsidian – A glassy volcanic rock, dark colored with conchoidal fracture.
Offset –The displacement between two geologic units that used to be contiguous before faulting.
Onyx – Straight, color-banded chalcedony. A Mexican onyx is essentially a carbonate (calcite), not silica.
Oolitic – It is a texture of ‘fish-egg like aggregates’. It can grow from oolith.
Opal – It is solid, hydrous silica jell. It can occur in various low-temperature conditions in veins or nodules. It can appear in volcanic or sedimentary terrain. The best of it exhibits opalescent luster. Most are in white tinted with other colors. If heated, its value can decrease through dehydration.
Overburden – The material above a layer of interest.
Parallelepiped — A six-sided solid with each side being a parallelogram.
Parallelogram – A plane geometry enclosed by two pairs of parallel line segments.
Parental rock – A source rock from which a later-formed rock has derived.
Patina – A thin, weathered coating on the surface of metals or stones. It differs from desert varnish in that the patina originates from weathering of the object itself, not external addition as in desert varnish.
Pebble – A rock fragment having a diameter between 4 and 64 mm.
Petrified wood – Fossil wood of which the wooden material has been replaced by silica.
Playa – A dry lake in the desert. Most of the time, it is dry.
Post-depositional – Something happened after a depositional event.
Pulverized – Making material into powder by natural grinding.
Pyrite – An iron sulfide. It is usually yellow with cubic or octahedral crystalline form. It is nicknamed fool’s gold.
Pyroclastic – Clastic rocks formed through volcanic activities.
Pyroxene – A group of dark rock-forming minerals that are mainly of magnesium, calcium, aluminous silicates. Unlike the amphibole, it lacks the hydroxyl component and its cleavages, if seen, frequently intersect near 90 degrees, rather than 56 or 124 degrees. Jadeite (hard jade) belongs to pyroxene group. Pyroxenite is a rock mainly consisting of pyroxene.
Quartz – A crystal form of silica.
Quartzite – A metamorphic rock that has been transformed from sandstone at high temperature and pressure without going through melting.
Sand, sandstone – Sand is a rock fragment of which the diameter lies between 1/16 and 2 mm.
Schistosity – foliation that is formed by parallel arrangement of minerals, for example, mica.
Semi-conductor – A solid substance that simultaneously acts like an electrical insulator and conductor, depending on the direction of electric current flow.
Schorl, tourmaline – Schorl is dark, black tourmaline. Big tourmaline crystal makes good museum display and some can be of gem quality.
Sericite – A short, flaky, white mica.
Shale – A sedimentary rock consisting chiefly of clay. It is finely layered and splintery.
Silica – Silicon dioxide.
Silicate – A mineral consisting of anion SiO4 (or other forms) and metallic ions.
Silicified fossil, wood – Fossil or wood of which the original organic constituents have been replaced with silica.
Silt – A rock fragment with size ranging from 1/256 to 1/16 mm in diameter.
Sink hole – A hole resulted from dissolution of limestone, salt, or gypsum.
Simulation — A computer modeling that is purported to explain or simulate a physical system.
Sorting – A measure of the uniformity in grain size distribution of sediments.
Spheroid – It is like a sphere with one axial length greater (prolate) or shorter (oblate) than the other two.
Stratigraphy – study of rock strata – its correlation, condition of formation, and evolution.
Streak – Long, narrow lines on the surface of rocks, distinctive with different colors.
Striae – Linear, parallel, short, slight ridges or grooves on a flat surface. (Singular: stria.)
Suiseki – The art of stone appreciation in Japanese.
Tetrahedron – A solid with four triangular faces.
Unconformity – It is a stratigraphic boundary that separate rocks of different ages but it should not be a fault contact surface.
Varnish, desert varnish – It is a thin film cover of rock (a few micrometers). It is hard, shining, or glazed iridescent with various colors. It is mainly a coating mix of clay, iron & manganese oxides, with some microbial. It is an add-on to stable, hard rocks in very dry desert environment.
Vein – A thin, sheet-like intrusion into crevices in rocks.
Ventifact – A stone shaped, worn, faceted, cut, or polished by wind blasting in the desert or other very windy environment.
Vermiculate – An adjective to describe a stone that has the appearance of being ‘gnawed by worm’.
Volcanic ash – An ash-like rock that has been spewed into the air by volcanic eruption before its falling to the ground.
Vug – A cavity in a rock.
Wonder rock – Nevada wonder rock – a lithified volcanic ash (rhyolite) with multiple bands of diffusion relics (Liesegang bands). It does not have to originate from Wonder, Nevada.
Wash, desert wash – A broad, dry gravely bed along an intermittent stream in the desert.