Silicon dioxide is commonly known as silica, as in silica sand. In elemental form, silicon is the essential material for semi-conductors used in many technological and scientific applications. As an oxide, silica is the most important material used to make glass and solar panels.
By weight, silicon is the third most abundant element (15%) in the Earth (iron 32% and oxygen 30%). Most of silicon appears in the crust and mantle (the top 2,900 km of the Earth). In the natural state, silicon does not exist in elemental form; it co-exists with oxygen in silica and in silicates with other additional metallic elements. Quartz is a crystalline form of silica. Most quartz appears as small grain in various types of rocks. Non-crystalline or cryptocrystalline silica includes chalcedony and its varieties. Opal is a hydrated cryptocrystalline form of silica.
Igneous rocks originate from solidification of magma (molten rocks). Their constituent minerals form at different stages of magma cooling, and quartz crystallizes during the later stages. Pure quartz is commonly crystal clear but large museum-quality crystals are rare. Inclusion of other elements or bubbles can alter the appearance of quartz crystals, some becoming colorful while others becoming dull. Also during the late stages of magma evolution, aqueous fluids with large amounts of dissolved silica can fill cracks in existing rocks because of their high mobility, ultimately manifesting in the form of quartz veins or dikes (namely quartzolite for high quartz content, >90%, but the term is not used hereafter). Under extremely high temperature and pressure in the Earth’s interior (depth > 1000 km), silicates decompose into exotic crystals of silicon dioxides, which no one has ever seen except in meteorites or in the laboratory simulation.
On the Earth’s surface, rocks are exposed to agents of physical erosion by rain, snow, ice, and wind as well as chemical weathering. Quartz-bearing rocks can physically disintegrate to yield discrete quartz grains as sand. Chemical alterations of silicates can release silica into solution, which eventually precipitates as quartz and other crystalline or amorphous varieties. The amorphous are exemplified by chalcedony and chert. Chalcedony becomes: agate characterized by curved, color bands; onyx with straight, color bands; or opal (hydrated) without banding but some with opalescent luster. Jasper is a variety with translucent reddish appearance. Chert and flint are depositional products. Dissolved silica can also replace buried animals or plants to form silicified fossils, for example, petrified wood. (Note that chalcedony can precipitate from hydrothermal fluids too.)
Quartz is the most common component of sand. When consolidated and cemented, sand becomes sandstone (commonly it also bears minor amounts of other components). Through deep burial or in contact with magma, sandstone can recrystallize without going through melting at high temperature and pressure to form a hard, quartz-rich metamorphic rock called quartzite.
This chapter deals with silica from various source rocks. My samples include quartz crystals, amorphous opal/chalcedony, and brecciated (fragmented) and re-grouped quartz aggregates. Some are ‘true’ quartz and many are quartzite. In particular, the roles of water and wind in the shaping of desert quartz are explored with examples.
I was awed at the first sight of this well-polished and varnished stone from a to-be developed housing tract in a desert wash. It is a piece of conglomerate, I thought, but immediately I realized my mistake. Conglomerate, a consolidated assembly of rock fragments, is highly unlikely to have a uniform composition of quartz grains only. The grain shape and geometric arrangement also say otherwise. In texture, it is porphyritic (large crystals entrenched in fine matrix). Could it have originated from a rising and cooling magma deep in the Earth? This assertion would imply that the coarse grains crystallized at earlier time of slow cooling are imbedded in the fine-grained matrix of the same material formed later in a rapidly cooling magma – an unlikely scenario. (Note that crystals generally grow bigger in a slowly cooling magma while small crystals form during rapid cooling.) After I have stored the stone away and forgotten it for almost three decades, my puzzle became clear one day. The rock was first shattered by an unknown natural mechanical force, then, the resulting cracks were invaded by siliceous fluid, and finally the fluid settled and appeared as background matrix. The stone is here coined as a piece of cataclastic quartz. But unlike clastic sediments assembled through transport, here the quartz aggregates are in-situ broken pieces through a catastrophic-like event. See Figures 1-2 and 1-14 for more similar pieces.
This quartz stone is characterized with crisscrossing fissures. However, it was not sufficiently brecciated (fragmented) as to have silica-rich fluid infiltrated into the fissures. Unlike its cousin in Figure 1-1 with coarse quartz grains immersed in microscopic grains of matrix, it does not have fine-grained background matrix although staining along the fissures did occur.
If the fragmentation happened before the stone’s separation from its parent rock, we might infer that, this stone was farther from the center of brecciation. See Figure 1-14 for pieces in gradation from intact to fragmented quartz stones.
Those samples do not have to be fractured by catastrophic events. Fracturing in vein quartz, as appears likely for the stones in Figure 1-1 and this one, can also occur due to thermal contraction during post-crystallization cooling, or pressure-release expansion due to erosional removal of overburden.
People can imagine something out of non-existence. We can fantasize an object into something else. Place names or scenic landmarks are often poetic. Here is my poetic truth: a mother bear proudly holds her baby on her back and wanders in the vast desert. This scene or illusion took me back seven decades to when I was a first grader: I wandered twice daily in the countryside with my baby sister strapped on my back in search of my mother to breast-feed my sister. Back then, after my father disappeared into political prisons of the Nationalist Chinese in Taiwan, my mother had no choice but to become a migratory day laborer to raise four kids.
My grandson JJ picked up the two pieces of coral on either side of the quartz ‘bear’ from a beach and placed them into my hands at the celebration of his first birthday. I am grateful to the USA for my opportunity to succeed, something that I had never dreamed or imagined as a financially stressed boy.
Here, two of the three wind-polished quartz stones have windows or pierced holes. How did the holes form? Wind-sand blasting or water dripping cannot pierce through these two stones, which were lying flat naturally in the desert, because wind or water cannot focus to drill holes vertically from both the top and bottom surfaces as evidenced by solution depressions on both surfaces.
Water can drill holes, as by mechanical erosion that occurs frequently on pavement under eaves. But such dripping process cannot drill those holes: the little amount of rain in the desert will not provide the repeated impacts of drops at one spot necessary to accomplish the drilling. Similarly, chemical weathering by naturally occurring carbonic acid in the rain water has little to do with drilling; because of high evaporation rate in the desert, the rain drops evaporate before they can corrode quartz, which is very stable chemically and hence very resistive to weathering.
Nevertheless, water has played a crucial role. Even in the desert, minuscule amount of moisture accumulates around micro cracks such that ice forms at temperature below freezing point to seal crack openings. Water inside the ice-sealed cracks expands as the interior temperature declines below 4oC. Cooling-induced expansion creates stress to widen and lengthen the cracks. The enlarged cracks suck in more moisture when the ice thaws and more cracking occurs through such feedback enhancement. Cracking excavation associated with repeated cycles of water freezing and ice thawing eventually tunnels bi-directionally through the ‘front and rear’ faces.
The beauty of these two desert quartz stones arises from water carving into wind-polished surface.
Water plays three powerful roles: First, flowing water during an occasional flash flood, loaded with suspended particles, will abrade anything along its flow path. The enormous energy of the raging water can also tumble the pebbles, cobbles, and even boulders, and grind them down from angular to rounded shapes. Second, water dissolves carbon dioxide from the atmosphere or soil to become carbonic acid or more likely bicarbonic acid. This weak acid, although still weak enough for us to touch or drink, can be very corrosive over geologic time. The effect is especially conspicuous in the limestone country where spectacular caves and karst landscape attract millions of visitors yearly. But can naturally acidic water corrode quartz, the most stable natural compound on Earth’s surface? Think about why chemists use glass bottles to store acids.
Third, as I argued for hole-piercing of quartz in Figure 1-4, water also plays a critical role because of its behavior of cyclic freezing-thawing in the desert. The role applies only if the cracks have been sealed by ice forming so as to close the crack system during thermal expansion and contraction; an open system that allows water to seep in and out freely will not work well.
This stone is a piece of milky quartz coated with a beautiful, natural, shining, dark-gray varnish of manganese oxides. The piece is held on the stand with two pieces of coral. Variations of darkness, shininess, and smoothness suggest different durations of surface exposure to the harsh desert environment. Light blue-gray patches mark the less coated spots (more common on the rear side). Mottling or scallop-shell-like features have developed along micro cracks, which resemble the conchoidal fractures that form when a piece of flint (chert), jasper, obsidian, or quartz crystal is broken. Cracks also occur on the exposed surface, as a result of expansion and contraction associated with daily and seasonal heating and cooling in the desert over hundreds of thousand years.
The shining black coating indicates its inorganic origin; biological activity tends to yield patchy or streaky dull-black coating. The air-exposed surface was coated (varnished) repeatedly over a long period of time when trace amount of manganese-bearing water dried out by evaporation. It has also been claimed that the desert varnish can be coated by airborne mineral and biogenic ingredients – sort of ‘natural pollution’.
These four different stones represent variations of silica products formed under different environments. Their current appearances have resulted from erosion and weathering by water. The brown crusts are added on coating, not patina or weathering product of quartz.
Standing on the yellow sand is a chunk of quartz that has endured low-temperature hydrothermal dissolution as if it were gnawed by worms. The fossil-bearing piece on the blue sand is former limestone that has been highly silicified; the brachiopod was fossilized during the Paleozoic Era, hundreds of millions of years ago.
The rock on the red sand exhibits features of both growth and decay. The ‘hives’ or cavities in the front are symptoms of erosion or decay while the smoothly arched lid in the upper rear seems to be indicative of local growth. The ‘skull’ on the center post is chalcedony, a form of amorphous quartz. Numerous thin, short, rectangular septa inside the skull are indicative of chalcedony accumulation and also, perhaps some quartz growth.
Figure 1-8a and b
Three contrasting hues appear on this quartzite stone. Its bottom face has been stained red by ferric oxide, presumably derived from the soil in which the rock was partially buried because quartzite itself does not yield red pigment. Grooves or clefts occur prominently around the sides. The part of the rock that jutted out of the soil has been wind-polished (wind blew right to left, as implied by asymmetric traverses across the ridge located near the mid-line) and coated with a veneer of transparent varnish. Dissolution relics appear on the tope surface, revealing the true white color of the stone.
Figure 1-8b: (Rear view). Numerous dark streaks appear along clefts which develop parallel to the bedding. These streaks are likely microbial-generated organic matter, not coatings of inorganic manganese dioxide. The presence or absence of dark streaks on the lateral faces is related to exposure of sun light and hence availability of moisture for the microbial (or lichen) to grow. Also, on the front face (Figure 1-8a), the streaks appear on the lee side (left face) of the asymmetric ridge, which was shaped by wind abrasion.
Here are three parallelepipeds of quartzite and one drill core of marble. Let’s focus on the big snow-white stone only.
Looking straight down, we can see a parallelogram whose four sides intercept at angles of either 60 or 120 degrees. The sharp corners with the acute angles (< 90 degrees) were worn or damaged as the stone suffered splintering or erosion after separation from its parent rock body. In contrast, the corners with the blunt, obtuse angles (> 90 degrees) are better preserved. The faces, which constitute the vertical sides of the parallelepiped, join at 90 degrees with the horizontal top and bottom surfaces of the stone.
In the following, I’ll invoke reduction in lithostatic (or overburden) pressure and cooling contraction to account for the two horizontal and four vertical joints (six parallelograms) that bound this parallelepiped stone. Two questions: First, why do the horizontal faces join the vertical faces at 90 degrees? And second, why do the vertical faces join each other at either 60 or 120 degrees?
Generally the joint pattern reflects material inhomogeneity and anisotropy (variation of property with direction) as well as external stress that causes disjoint in the parent rock body. A joint is a fracture without appreciable displacement along the surfaces of breakage; else, a fracture with appreciable displacement is known as a geologic fault.
Here is the story: Originally a set of flat layers of quartz sand in the seafloor was buried to ever increasing depths by continuous deposition atop a sinking basin floor. Those layers were first lithified to make sandstone and then metamorphosed to form quartzite. Sometime after the metamorphism, a regional tectonic uplift happened. Associated with the uplifting, erosion at the ground surface reduced the overburden pressure on the quartzite and consequently, the quartzite ‘popped’ to form horizontal fractures (tensional joints), which followed the original layering (an inherited weak interface).
The reduction in vertical overburden pressure should be accompanied by similar amount of pressure reduction in other directions (analogous to hydrostatic pressure reduction). As such, one would expect a pattern of concentric expansion fractures to develop. But it does not happen because horizontal expansion is constrained laterally by the surrounding rock bodies and then, the lateral constraint allows easier and faster vertical expansion to generate horizontal fractures. Thus, the early development of horizontal fractures, as resulted from uniaxial decompression induced by erosion at the ground surface, prevents concentric fractures from forming and facilitates vertical joints.
Now, why do the vertical joints meet at angles of 60 or 120 degrees? Theoretically, an intact, isotropic, homogeneous rock tends to crack along plane of maximum shear stress, which occurs midway (45 degrees) between the maximum and minimum stress axes to hypothetically create 90-degree joints. Owing to frictional resistance to fracture sliding, the fracture orientation shifts, in this rock sample, as to have a conjugate pair of joints at 60 and 120 degrees.
Additionally, let’s ponder another plausible cause. As the rock body rises from the deep interior, it cools and contracts. Cooling contraction induces the rock to shrink whereas pressure reduction causes it to expand. The two factors exert opposing stresses and each can cause horizontal fracturing; but contraction and expansion do not cancel each other because both do not have equal strength and do not operate simultaneously at their respective highest stress levels. Which is the dominant factor? I believe, for a given set of material properties, the predominance varies with time and depends on rates of erosion and uplift as well as cooling. To resolve the issue, a painstaking field observation would have to be made in conjunction with numerical simulation.
In this dull-looking piece of layered quartzite, two zones (B & D) recede by erosion more than the rest of the stone. Here are two possible but mutually exclusive stories for the excessive recession: (1) the two represent depositional hiatus prior to the quartzite-forming metamorphism; or (2) the two have resulted from expansion cracking by reduction of overburden pressure and subsequent crack filling after the metamorphism.
At the beginning, a sequence of clay (grey) and fine sand or silt (white) had quietly accumulated under seawater on the continental shelf for millions of years. The layered structure was dictated by the nature of the sediment supply, for example, rivers of which the carrying capacity varied seasonally and secularly in response to weather or climate changes; and hence the difference in clay and sand content. With continuing sedimentation, sediment became progressively buried and compacted. Upon sufficient burial, the sediments were heated up and the pore waters became increasingly saturated with dissolved silica, helping to cement the sediments together to form siltstone. At elevated temperature and pressure with increasingly deeper burial, the siltstone was eventually metamorphosed to the hardened quartzite – the stone presented here.
However, during an otherwise continuous deposition, the sequence was punctuated with a brief hiatus (marked by disconformity) as shown by one intervening layer (Zone B) with fine grained quartz crystals (about 1 mm in diameter) and another layer (Zone D) with distinctive, light brown, fine particles near the top of the section. Both hiatus zones recede more than the average for other zones in this stone, indicating their lesser resistance to erosion. In essence, pre-existing weakness before the rock was metamorphosed is the cause of differential erosion.
The stone’s story could have ended here if one thin, slanting quartz ‘VEIN’ were absent. An alternative story is therefore entertained here for the presence of the vein – my smoking gun.
The vein runs across Zone C between the two recessed zones (B and D), representing a post-depositional filling of a minor fracture that cuts across Zone C but does not extend upward through Zone D. Puzzlingly, the vein seems to have jumped across Zone B and extended halfway into Zone A (bottommost). These observations beg the question: When did the vein (or fracture) occur? And, is there an alternative story to the hypothesis of depositional hiatus for the occurrences of Zones B and D? In brief, can the occurrences of B & D be post-metamorphism?
Assuming Zone A is older than Zone E, the vein had appeared after the birth of Zone C but before the appearance of Zones D and E. The vein’s halfway extension into Zone A means that it appeared after Zone A had formed, asserting that the vein is older than D but younger than C. Its absence in Zone B suggests a portion of the vein has been wiped out by the appearance of Zone B, which accordingly is younger than the vein and Zone C. How?
Take a different view at the reconstruction of events. Instead of regarding Zones B and D as records of hiatus in an otherwise continuous depositional environment, consider the two zones as alien intruders (or fillers) in an evolving environment of post-deposition and -metamorphism.
Prior to the insertion, the sequence of events was: A, C, Vein, and E. The removal of overburden by erosion reduced the pressure on rocks, and the once-compressed rocks relaxed and expanded accordingly. Subsequently, a fracture developed between Zones A and C; and a silica-rich fluid found its way to fill the newly opened fracture to form Zone B. Fine-grained quartz eventually crystallized from the fluid. This hypothesis explains why Zone B is devoid of the quartz vein, is younger than the vein, is more prone to erosion (relative to metamorphosed Zones A, C, and E), has not been metamorphosed, and has retained a unique zone of visible, distinctive quartz crystals. In short, Zone B is both younger than its underlying Zone A and overlying Zone C.
Next, Zone D followed a horizontal fracture that developed after the fracturing for Zone B. It appears to be a product of near ground-surface filling because of its yellowish brown color, mud-like texture, and lack of visible crystals. My view of Zone D as a post-metamorphism, secondary filling explains why it is younger than its overlying but metamorphosed Zone E, has not been metamorphosed into quartzite, and is less resistive to erosion.
The jagged upper and lower boundaries for either Zone B or D are consistent with the contention that they are fracture interfaces, not inherited from the originally smooth sedimentary contacts. In summary, the age sequence of the various components is A, C, vein, E, and then B and D.
A geode is a sub-spherical nodule with its exterior wall (shell) made of chalcedony (an amorphous form of solid silica) and its inner cavity filled or partially filled with inward growing crystals – mostly varieties of quartz and less frequently calcite. Geodes occur in beds of limestone, shale, or volcanic ash. Sometimes they also appear in volcanic lava cavities (vugs) or tubes, which were once occupied by gases as part of the lava flow.
This geode has two distinctive stages in cavity filling. First, it grew inward from the chalcedony walls of the cavity, as characterized by bands of chalcedony that follow the irregularly-shaped cavity walls. Such wall-parallel infilling was then followed by deposition of horizontal layers of chalcedony but the deposition ended when the siliceous fluid was exhausted. Questions: Did the filling begin from a fixed amount of fluid? Was the fluid being replenished during deposition?
The horizontal banding reminds me of layered deposits in old municipal water pipes that have distributed hard water (i.e., water with high content of calcium carbonate or bicarbonate). The layering in water pipes actually resembles the deposition of onyx calcite, which appears in cold water of caves, rather than the chalcedony banding of geodes that form at relatively higher temperatures.
Figure 1-12a and b
This piece of quartz is shaped roughly like a parallelepiped. Poorly polished, it exhibits three color bands. The sky-facing top surface (left figure) is light reddish brown, with black mottling in pits or grooves. The red face (right figure) was ground-facing and is free of mottling. The light, whitish band on the side faces ranges from unstained to heavily mottled or stained. Overall the stone lacks the shinning sheen of desert varnish.
The brown stain on the top and the red stain under the bottom are indicative of different staining agents or represent different extents of staining. We notice the two types of stain appear to have ‘spilled’, respectively, downward from the top surface and upward from the bottom surface.
The mottle is dull and dark, indicating a likely biologic origin (algae or bacteria, or incipient lichen development with symbiotic algae and fungi). The ‘algae’ appear to have thrived on the sunlight because the ground-facing bottom is mottle-free. This speculation, however, is contrary to what was said about the occurrence of dark streaks on the stone in Figure 1-8. It is also noted that the mottling here adheres to the base of relief, grooves, or pits, unlike the dark streaking in Figure 1-8 that follows bedding crevices or clefts.
Some scientists believe desert varnish originates from lichen that absorbs moisture and nutrients from the desert air (like air plants in humid areas). I believe the shining varnish is of inorganic origin whereas the dull type is of organism, which thrives on either strong sun light (lichen) or in the shade with slightly high moisture content (microbial). The dark varnish on the Martian rocks observed by orbiting satellites seems to favor inorganic origin. Also, see Figures 3-4, and 3-5 for surficial pigmentation.
Noticeably, the two quartz crystals bear needle-like rutile and are otherwise transparently clear. The murky features reflect brown coating on the rear unpolished surface, which exhibits protruding, pyramidal studs of smaller quartz crystals. Some small crystals are visible at the lower-left corner. These two specimens have been polished to reveal their interior features. Extraneous reflections from multi-facets of the crystal cause ghost features of greenish yellow on the top and bluish sheen on the right facet. A quest: how did the rutile maintain its needle straightness within crystallizing quartz?
This multi-faced semi-transparent quartz crystal is an aggregate of several single crystals. The stone has been cut but not polished. The green mineral is tourmaline. Its rectangular dark green core is enclosed within a like-shaped light green envelope, reflecting change of trace element concentration in the tourmaline. Note also the sharp boundary between the dark and light green. The contrast results from geochemical differentiation, rather than diffusion from high to low concentrations.
Each crystal is a hexagonal prism but its six facets are not of equal size. The smoky quartz is columnar (height, 19 cm) while the transparent one is plate-like (15x3x7 cm). At the base of the smoky quartz, the brown crystals are orthoclase. All prismatic facets are striated. Being a common diagnostic feature of natural quartz crystals, the striae mark the ripple in crystal growth. Note that the crystals are terminated with six stria-free pyramidal faces. Why do the pyramidal faces lack in visible striae? How does a growing crystal know when to terminate its end without striated marking? Could the crystal grow along its hexagonal axis by accreting (adding) pyramidal envelops (thin sheets)? The striae are birth vestiges of stacking those incremental thin top envelops.
This cataclastic quartz is moderately polished, stained, and varnished with a small dull patch of calcareous material at the mid-height of the central groove. This patch formed near the ground surface, long after the brecciation of the rock (Compare it with Figures 1-1 and 1-2).
This fractured quartz is well polished and varnished. It is severely shattered and shows some brownish staining along cracks, a relic of fluid infiltration.
These quartz pieces reveal different degrees of shattering, less shattered pieces in the middle, and one piece at intermediate stage of brecciation (left).
My sons CT and CY collected this geode from a volcanic ash bed in the desert during their pre-school days in the late 1970s. The greenish hue reflects alteration of volcanic ash into chlorite. It would be necessary to break or cut into the geode to find out what are inside. For example, see Figures 1-11 and 1-18.
This geode is essentially a lava tube with in-growth of purple amethyst (a variety of quartz) encased by a hull of milky quartz and chalcedony. The large white ‘spikes’ scattered on some amethyst are calcite. How did the big cavity come along? Was it a former huge volcanic gas chamber filled with all the needed fluid for the crystal growth? Here is another scenario.
When a rivulet of lava spills downslope, its exterior cools and solidifies rapidly to form a crust or shell; inside the shell, the liquid lava continues to flow but leaves behind a hollow tube when the supply of lava is interrupted. Later, a silica-rich fluid enters the hollow tube and the silica precipitates on the inner surface of the tube, first as chalcedony or later as amethyst. Occasionally at a later stage of development, a calcium-carbonate-bearing fluid enters the tube to form calcite. (It could represent residual fluid after silica precipitation rather than a new fluid inflow.) Apparently the amount of calcium carbonate as calcite is insufficient to fill the entire tube. Why doesn’t calcite precipitate as small crystals throughout the inner tube? The scattering of spiky calcite indicates that the crystals grow from stochastically seeded tiny crystals over some amethyst, as triggered perhaps from earth shaking of a fluid oversaturated with calcium carbonate.
All specimens are naturally rounded, polished, and varnished.
The two pieces of quartz are works of natural water carving, as aided by water freezing and ice thawing.
Both black pieces of quartz are examples of ‘miniature landscape’ carved by water and wind, and they are also very well polished and varnished naturally. This pair and some others are good for suiseki (the art of stone appreciation in Japanese).
A papaya-colored ‘upward concave arc’ demarcates the green copper-bearing malachite from the waxy-looking milky chalcedony. The green is likely enclosed in the chalcedony and it is exposed through a ‘window’ of missing chalcedony cover. The green consists of the cracked, weathered malachite and a lumpy, grayish green mineral (marked with red dots). The latter is magnetic and is assumed to be malachite-tinted magnetite or pyrrhotite (the two are iron oxide and sulfide, respectively).
The chalcedony ranges in form from massive to sheet-like (marked by Q), and in color from white (on the right side, unseen here) to milky white with bluish tint in between. Thin malachite seam underlies the sheet or encircles small chunk of chalcedony. Funnel-like pits (one cm deep) appear on the front face of the massive chalcedony; some of the pits are filled with unknown, black minerals. Near the bottom, there exists one calcite crystal (marked by C). The rest of the chalcedony is covered with a not-easy-to-remove veneer of papaya-colored silica; and atop the veneer, there lies scattered white, calcareous caliche, which is likely acquired in the desert wash, not at its parental site.
It appears that an ore deposit of malachite had been brecciated and the scattered breccia were cemented by siliceous solution as chalcedony and the residual solution of malachite filled the cracks or grain boundaries in the chalcedony.
The simplicity of this quartzite piece makes it ideal for suiseki display.
Various forms of silica can appear inside geodes. Here are three forms of white chalcedony: fibrous (lower left), layered (ceiling & floor), and botryoidal (right half). A half-circle on the floor seems to indicate where the silica-bearing fluid had oozed during the last stage of chalcedony precipitation. (The dark spot next to it is an empty crack of this broken specimen.)
All except the greyish white one (upper left) are transparent, hexagonal prisms with terminal pyramidal facets. Striae (linear, repetitive, slight ridges) appear on all prismatic facets but the pyramidal facets are as usual stria free. The two right-most crystals are citrine (yellowish brown) and amethyst (purple) respectively. Secondary, banded, brownish coating has tarnished an otherwise transparent crystal but an ellipse-backlight makes the jointed crystal to appear like a twin pagodas. The piece at the lower middle is actually a two-in-one jointed crystal. The greyish white piece (upper left) is fairly common and you can spot pieces like that in the field or some mining dumps but for the rest, you have to purchase them or be very lucky to find such clear, transparent crystals in rock crevices, caves, or mines.