Quartz is one of the most common minerals found in the Earth's crust. If pure, quartz forms colourless, transparent and very hard crystals with a glass-like lustre. A significant component of many igneous, metamorphic and sedimentary rocks, this natural form of silicon dioxide is found in an impressive range of varieties and colours.

The Si analogue of pertoldite.

Macro- and Cryptocrystalline Quartz

Quartz occurs in two basic forms:

1. The more common macrocrystalline quartz is made of visible crystals or grains. Examples include rock crystals, the grains in sandstone, as well as massive quartz, which is composed of large crystallites without any crystal faces, such as vein quartz.

2. Cryptocrystalline quartz or microcrystalline quartz is made of dense and compact aggregates of microscopic quartz crystals and crystallites. Examples are agate and chert. The different types of cryptocrystalline quartz are colloquially subsumed under the term chalcedony, although that term has a more strict definition in scientific literature. It is worth mentioning that most chalcedony contains small amounts of another SiO2 polymorph, moganite, so it is not always pure quartz.

Quartz Varieties

Quartz crystals or aggregates that share certain peculiar physical properties have been classified as quartz varieties with specific "trivial names".
The best known examples are the colored varieties of quartz, like amethyst or smoky quartz. Still, there are also trivial names for specific crystal shapes, aggregates and textures, like scepter quartz, quartz gwindel or quartzine. Because there are no canonical rules on naming or defining quartz varieties like there are for minerals, the definitions of some quartz varieties are precise and generally accepted, while the definitions of others vary considerably between different authors, or are rather fuzzy.

Mindat Classification of Quartz Varieties
On Mindat, macrocrystalline quartz and its varieties are listed as quartz and varieties of quartz.
Cryptocrystalline quartz and its varieties are listed as chalcedony, like "Quartz (Var: Chalcedony)", or as variety of chalcedony, like "Chalcedony (Var: Agate)".
More about the specific properties of chalcedony and its varieties can be found at the respective mineral pages.
Note that, contrary to minerals, the definitions of varieties are not mutually exclusive in the sense that no mineral can be another. A single specimen can be correctly classified as several varieties.

Structure of Quartz

The structure of quartz was deciphered by Bragg and Gibbs in 1925 (for a review of the structure and symmetry features of quartz, see Heaney, 1994). Its basic building block is the SiO₄ group, in which four oxygen atoms surround a central silicon atom to form a tetrahedron. Since each oxygen is a member of two SiO₄ groups, the formula of quartz is SiO₂. The SiO₄ tetrahedra form a three-dimensional network, and many mineralogy textbooks classify quartz as a network silicate or tectosilicate.

Quartz can be thought of as being made of threefold and sixfold helical chains of SiO₄ tetrahedra that run parallel to the c-axis. Figure 1 shows two representations of a threefold SiO₄ helix and its relationship to the quartz unit cell: to the right a ball model with red oxygen and white silicon atoms, to the left a tetrahedral model, with the corners of the tetrahedra at the position of the oxygen atoms.

Six of such helices are connected to form a ring that surrounds a central channel, which runs parallel to the c-axis, sometimes called "c-channel". The SiO₄ tetrahedra around the central c-channel form two independent sixfold helices. Figure 2 shows two views of the corresponding structure: looking in the direction of the c-axis in the top row, and looking in the direction of the a-axis in the bottom row. Like quartz crystals, the ring is six-sided but has a trigonal symmetry. The large channels are an important structural feature of quartz because they may be occupied by small cations.

You can explore the crystal structure of quartz with the interactive tool JSmol further down this page.

Handedness of Quartz Crystals

A helix is either turning clockwise (right-handed) or counterclockwise (left-handed). Due to the helical arrangement of the SiO₄ tetrahedra, the atomic lattice of quartz possesses the symmetry properties of a helix: Quartz forms left- and right-handed crystals, whose crystal structure and morphology are mirror-images of each other.

In a crystal with space group P3₁21 (right), the sixfold helices turn counter-clockwise (left) and the threefold helices clockwise (right).
In a crystal with space group P3₂21 (left), the sixfold helices turn clockwise (right) and the threefold helices counter-clockwise (left).
For a thorough review of the symmetry features of quartz, see Heaney (1994).

The crystallographic form of quartz that is characteristic of its symmetry properties is the trigonal trapezohedron. The position of the faces of the positive trigonal trapezohedra on the crystal reflects the handedness of the structure of the crystal. The figure to the right visualises the relationship between the handedness of the six-fold helices and the position of the faces of the positive trigonal trapezohedron (x - orange) and the trigonal bipyramid (s - blue). Unfortunately, these faces are not present on all crystals, and often it is not possible to determine the handedness of a crystal from its morphology.

Quartz is optically active: the polarisation of a light ray passing through a crystal parallel to the c-axis will be rotated either to the left or the right, depending on the handedness of the crystal (Arago, 1811; Biot, 1812; Herschel, 1822). The relationship between the handedness of the crystals and the symmetry of the structure, and hence the optical rotation, was determined by de Vries (1958).

The following table lists how symmetry, morphology and optical behaviour are related.
Note that the morphological handedness as expressed by the position of the trapezohedral and bipyramidal faces x and s does not match the symmetry's handedness:

Space Group Handedness of sixfold helix Handedness of threefold helix Indices for x- and s-forms Position of x- and s-face Rotation of polarization of light Left-handed Quartz P3121 (right) structure: left (counter-clockwise) symmetry: 31 (right) structure: right (clockwise) symmetry: 31 (right) x {6 1 5 1} s {2 1 1 1} left left (counter-clockwise) Right-handed Quartz P3221 (left) structure: right (clockwise) symmetry: 32 (left) structure: left (counter-clockwise) symmetry: 32 (left) x {5 1 6 1} s {1 1 2 1} right right (counter-clockwise)

Morphology

Quartz is found as individual crystals and as crystal aggregates. Well-crystallised quartz crystals are typically six-sided prisms with steep pyramidal terminations. They can be stubby ("short prismatic") or elongated and even needle-like. In most environments, quartz crystals are attached to the host rock and only have one tip, but doubly-terminated crystals are also found.
As a rock-forming mineral, quartz commonly occurs as sub-millimetre to centimetre-sized anhedral grains; well-formed crystals are uncommon. Secondary vein fillings of quartz are typically massive.

Quartz belongs to the trigonal-trapezohedral crystal class 32. Of the seven basic crystallographic forms of this crystal class, the hexagonal prism and trigonal rhombohedra are very common and determine the overall shape of the crystals. The trigonal bipyramids and trigonal trapezohedra are frequently found, but typically only as relatively small faces. The trigonal prisms, the basal pinacoid and in particular ditrigonal prisms are very rare (Frondel, 1962).

Quartz crystals show about 100 different crystallographic forms in nature (Frondel, 1962; Rykart, 1995). It is convenient and common practice to designate them with Latin and Greek letter symbols instead of Miller-Bravais indices. The following figure illustrates the relation of the common forms (sorted by abundance) to the faces found on quartz crystals. The most common combination of crystallographic forms in quartz crystals is r+m+z.



The handedness of quartz crystals can be determined easily from the positions of x faces, which are at the lower left or lower right corner of the r face (orange faces in Fig.5). With some difficulty the handedness can be determined from the position of the s faces (blue faces in Fig.5), which lie between the r and z faces: the s face often shows a fine striation that runs parallel to the edge of the r-face.
The bottom row shows a top view of the crystals. It not only shows their trigonal symmetry but also the chirality of the position of the x faces.


Macroscopic Structure of Quartz Crystals

In response to lattice defects, and reflecting their growth conditions, quartz crystals may develop two very distinct and mutually exclusive types of internal structure:
- Macromosaic Structure, sometimes called "Friedlaender Quartz"
- Lamellar Structure, sometimes called "Bambauer Quartz"

Individual crystals may possess both structural types, but the respective parts of the crystals grew at different developmental stages (Hertweck et al., 1998). It is sometimes claimed that all quartz occurs either as macromosaic or as a lamellar structural type. This is not correct.

The lamellar structure was first described by Weil (1931). The crystals contain layers that show an optical anomaly: they are biaxial. The layers are stacked parallel to the crystal faces in an onion-like manner and were found to be associated with a relatively high hydrogen and aluminium content (Bambauer et al., 1961, 1962, 1963). Lamellar quartz cannot be safely recognised without studying the optical properties of the crystal in a thin section.

Macromosaic quartz crystals have been described by Friedlaender (1951) and are composed of slightly tilted and radially arranged wedge-shaped sectors. They are recognised by the presence of sutures on the crystal faces, which are often confused with twin boundaries. Crystals with such a structure are found in pegmatite and miarole pockets and high-temperature alpine-type fissures.

Quartz Crystal Habits

Strictly speaking, the term "habit" is used to designate the overall shape of individual crystals, regardless of the crystallographic forms (crystal faces) involved. Confusingly, the definitions of some habits of quartz crystals do include specific forms. Many of the trivial names of these habits have been introduced and popularised by rock hounds in the Alps (for a good overview, see Rykart, 1995). The most important habits with trivial names (with synonyms in different languages in braces) are:
a) Normal habit ("Maderaner Habitus", prismatic habit): "typical" quartz crystals that are not or only slightly tapered.
b) Trigonal habit: Crystals with obvious trigonal symmetry, for example, because of missing z faces, or because of a triangular cross section, like in crystals with a Muzo habit (h).
c) Pseudohexagonal habit: Crystals with an even development of rhombohedral and prism faces.
d) Cumberland habit: Crystals with very small or absent prism faces, often bipyramidal.
e) Pseudocubic quartz (pseudocubic habit, cubic habit, cube quartz, "Würfelquarz"): Crystals with a dominant r or z form that look like slightly distorted cubes.
f) Dauphiné habit: Crystal tips with a single very dominant rhombohedral face.
g) Tessin habit ("Abito Ticino", "Tessiner Habitus", "Rauriser Habitus", "Penninischer Habitus", "Acute Rhombohedral Habit"): Crystals that are tapered by steep rhombohedral faces { h 0 i 1 }, Tessin habit in the strict sense is dominated by { 4 0 4 1 } and { 3 0 3 1 } faces. At the original locality, they possess a macromosaic structure.
h) Muzo habit: Crystals with prism faces that are tapered under the z faces because these are made of a succession of alternating m and z faces, and have a trigonal cross section at the crystal tips (Gansser, 1963).
Needle quartz (acicular habit): Crystals are greatly elongated along the c-axis.



Quartz Growth Forms

In addition to crystallographic forms and habits, many quartz crystals are characterised by peculiar morphological features that reflect different modes of growth during their development. Some of these "growth forms" are found at many different localities and - like habits - have been given "trivial names" (e.g., "cactus quartz", "gwindel"). Some of these are listed as varieties of quartz on Mindat. Among the more common and important growth forms are:
Sceptre quartz: Crystals with syntaxial overgrowth of a second generation tip.
Faden quartz: Crystals and crystal aggregates with a white thread running through the crystals. The thread is caused by repetitive cracking of the crystal during growth and consists of fluid inclusions.
Window or Skeleton or Frame or Fenster quartz: Crystals with frame-like, elevated edges of the crystal faces, usually with parallel grown blades that grow from the edges to the center of the faces in a window glass-like manner. Hopper crystals that correspond to skeleton-growth in the strict sense are rare.
Phantom quartz: Crystals in which outlines of the shape of earlier developmental stages of the crystal are visible because of inclusions or colour zones.
Sprouting quartz ("Sprossenquarz"): Crystals on which split-growth causes subparallel daughter crystals to sprout from the crystal faces
Artichoke quartz: A form of split-growth resulting in specimens with composite artichoke-like crystal tips.
Gwindel: Crystals elongated and twisted along an a-axis.
Cactus quartz or spirit quartz: Crystals whose prism faces are covered by small, roughly radially grown second-generation crystals.





Quartz Twins

Twinning is very common in quartz, but is often inconspicuous and difficult to recognise. Two types of twinning can be distinguished (data in tables from Jentzsch, 1867, 1868; Gault, 1949; Frondel, 1962):

1. Twins with parallel main crystallographic axes

Twinning Axis Twinning Plane Composition Plane Type Handedness of Domains Dauphiné Law [0 0 0 1] - {1 0 1 0} Penetration Twin R+R or L+L Brazil Law - {1 1 2 0} {1 1 2 0} Penetration / Contact Twin L+R Combined Law [0 0 0 1] {1 1 2 0} - Penetration Twin L+R

Dauphiné and Brazil law twins are very common. Most crystals, even if morphologically untwinned, contain at least small twin domains. Both types of twins can be found in a single crystal.

Dauphiné Law
Also called: Swiss Law, Alpine Law
Dauphiné law twins can be thought of as a merger of two crystals of equal handedness that are rotated by 60° around the c-axis relative to each other (Weiss, 1816). They are penetration twins composed of twin domains with irregular boundaries (Leydolt, 1855). The size and shape of the twin domains can vary, and the shares of the twin domains in a crystal do not have to be equal. The degree of intergrowth of the domains may increase during growth, starting from roughly triangular sectors at the base to complex irregular patterns at the tip of the crystal (Friedlaender, 1951). Twin domains are only rarely visible in natural crystals and normally need to be visualised by etching the surface or a polished cross-section (Leydolt, 1855; Judd, 1888). Electron microscopical studies reveal that on a small scale, the twin domains look like complex polygons with straight boundaries (Lang, 1965; McLaren and Phakey, 1969).

Dauphiné twins can sometimes be recognised by the position and arrangement of crystal faces, in particular, the x-faces. Because the rhombohedral faces are composites of r and z faces, they do not show the common size difference of the faces, and the crystals assume a pseudohexagonal habit.

Rarely, Dauphiné twinned crystals that lack one type of rhombohedral face (either r or z) - and that would display a trigonal habit if they were untwinned - show re-entrant angles at the tip that make them look like drill heads (for example, Schäfer, 1999).

Dauphiné twins are sometimes called electrical twins, because this kind of twinning reduces or even suppresses the piezoelectricity that is typical for untwinned quartz crystals, while their optical activity remains unaffected (Thomas, 1945; Donnay and Le Page, 1975).

Brazil Law
Also called: Optical Law
Brazil law twins can be thought of as a merger of a left- and right-handed crystal: they are penetration twins composed of left- and right-handed domains. Their twin boundaries are usually straight lines, resulting in a characteristic pattern made of straight lines and triangles (Leydolt, 1855). As with Dauphiné twins, the twin domains are usually not visible in natural crystals and need to be visualised by etching (Leydolt, 1855). The corresponding surface patterns on crystal faces are polygonal patches with straight boundaries, often triangular.

Brazil law twins that show the ideal arrangement of x and s crystal faces are very rare.

Many amethysts are twinned polysynthetically according to the Brazil Law: Parts of the amethyst crystals, in particular in zones under the r rhombohedral faces are composed of alternating layers of left- and right-handed quartz (Brewster 1823; McLaren and Pitkethly, 1982; Taijing and Sunagawa, 1990). The gauge of individual layers is normally less than 1 mm. The layered structure may be visible as a fingerprint-like pattern on rhombohedral faces.

Brazil law twins are sometimes called optical twins, because this kind of twinning reduces or even suppresses the optical activity typical for quartz crystals. Confusingly, and contrary to common belief, Brazil law twinning also reduces or suppresses the piezoelectricity of quartz crystals (Thomas, 1945; Donnay and Le Page, 1975).

Combined Law
Also called: Liebisch Law, Dauphiné-Brazil Law, Leydolt Law
It is not unusual for crystals to show Dauphiné and Brazil law domains in one crystal, and sometimes, crystals show x or s faces at positions that would indicate a special type of twinning. Electron microscopic studies show that when Brazil law twins are heated and develop new Dauphiné twin domains, their left- and right-handed domains do not share boundaries when they are rotated with respect to each other (Van Goethem et al., 1977), so Liebisch twinning seems to be energetically less favourable. Accordingly, Liebisch twinning is rare.


2. Twins with inclined main crystallographic axes (incomplete list)

Twinning Plane Composition Plane Type Inclination of c-axes Japan Law {1 1 2 2} {1 1 2 2} Contact Twin 84°33' Zinnwald Law {2 0 2 1} {2 0 2 1} Contact Twin 38°13' Breithaupt Law [1 1 2 1] {1 1 2 1} Contact Twin 48°17' Reichenstein Grieserntal Law {1 0 1 1} {1 0 1 1} Contact Twin 76°26'

Of the twins with inclined main axes, only the Japan law twin is common and well established, while for some of the others (including some that are not listed here) only a few and sometimes only one specimen have been reported and the existence of a twin law is questionable. The Reichenstein-Grieserntal Law is sometimes erroneously called "Esterel Law", which is the equivalent of beta-quartz.

Japan Law
Also called: Weiss Law, La Gardette Law
Japan law twins are the only common quartz twins with inclined c axes. The law was first found and described by Weiss (1829) on crystals from La Gardette, France, but the name "Japan law" became more popular after a great number of them were found in Japan. The c-axis of two crystals meet at an angle of 84°33', with two of the m prism faces of both crystals being parallel. The twinning plane {1 1 2 2} of Japan law twins corresponds to the flat trigonal bipyramid ξ (the Greek letter xi).
Japan law twins are contact twins (Sunagawa and Yasuda, 1983). The twin junctions often look jagged on the crystal surface, but are perfectly straight in the interior of the crystals, and form a thin plane that runs from the base of the crystal to the V-shaped indentation between the branches (Sunagawa and Yasuda, 1983). Electron microscopic studies revealed that the twin boundary also forms a perfect plane parallel to {1 1 2 2} (Lenart et al. 2012; Momma et al. 2015), but appears to be restricted to the initial growth periods of the crystal, extending only a few hundred micrometers, which has been interpreted as an indication of a formation as a nucleation twin (Lenart et al. 2012). The cause of the twin formation is still not understood.

Most Japan law twins are flattened, and often they are larger than untwinned crystals that accompany them. Depending on the handedness of the two branches of a twin, one can distinguish 8 different basic twinning subtypes that are also twinned according to the Brazil or Dauphiné law (Frondel, 1962), but the pattern of Brazil and Dauphiné twin domains can be very complex (Kozu, 1952).

Colored Quartz Varieties

Compared to many other minerals, quartz is chemically very pure; most crystals contain more than 99.5% SiO2. Nevertheless, varieties colored by impurities occur. These can be divided into two groups:

1. Quartz colored by trace elements built into the crystal lattice.
Only a few elements can replace silicon in the quartz lattice (substitutional positions) or are small enough to occupy free spaces in the lattice (interstitial positions). In natural quartz crystals, the most common ones to replace Si are Al, Fe, Ge, and Ti, whereas Li, Na, Ca, Mg and Fe often occupy interstitial positions in the "c-channels" mentioned under "Structure of Quartz". Of the substitutional trace elements, only Al, Fe and more rarely P are found to play a role in natural colored varieties. There are only a handful of quartz varieties colored by trace elements built into the lattice, sorted by abundance, with the more common ones first:
- Smoky quartz/morion
- Amethyst
- Citrine
- Pink Quartz / Rose Quartz
- Prasiolite

With the possible exception of some prasiolites and some citrines, the colour of these varieties is based on colour centres whose formation requires high-energy irradiation from radioactive elements in the surrounding rocks (O'Brien, 1955; Lehmann and Moore, 1966; Maschmeyer et al., 1980; Maschmeier and Lehmann, 1983). Quartz varieties based on colour centres are pleochroic, and their colour centres can be destroyed by heat treatment.
Note that individual quartz crystals may contain several coloured varieties, like an amethyst with smoky zones.



2. Quartz colored by inclusions of separate phases, for example minerals or fluids.
Because quartz crystals grow in many geological environments, they embed many different minerals during growth and assume the colours of the included minerals. Colours may also be caused by light scattering at finely distributed but colourless inclusions.
There are also trivial names for varieties colored by inclusions that have been found at many localities, like "prase", "ferruginous quartz" or "rose quartz". However, the definitions of these varieties are often rather fuzzy, and different authors use different definitions.

Occurrence of Quartz

Quartz is one of the crystalline forms of silica, the essential building material for all silicates, and quartz can only form where silica is present in excess of what is consumed in the formation of other silicate minerals.
Quartz may also be consumed during the formation of new silicate minerals, in particular at higher temperatures and pressures, and certain geological environments are "incompatible" with free silica and hence quartz.

Quartz as a Rock-Forming Mineral
Silica has been enriched in the continental Earth's crust to about 60% (Rudnick and Gao, 2003) by processes like magmatic differentiation and the formation of silica-rich igneous rocks (mainly driven by plate tectonics) and the accumulation of the physically and chemically stable quartz in sediments and sedimentary rocks. The oceanic crust's silica content of about 50% (White and Klein, 2014) in its igneous rocks is too low for quartz to form in them.

The largest amount of quartz is found as a rock-forming mineral in silica-rich igneous rocks, namely granite-like plutonic rocks, and in the metamorphic rocks that are derived from them. Under conditions at or near the surface, quartz is generally more stable than most other rock-forming minerals, and its accumulation in sediments leads to rocks that are highly enriched in quartz, like sandstones. Quartz is also a major constituent of sedimentary rocks whose high quartz content is not immediately obvious, like slates, as well as in the metamorphic rocks derived from such quartz-bearing precursor rocks.

Quartz Veins
At higher temperatures and pressures, quartz is easily dissolved by watery fluids percolating the rock. When silica-rich solutions penetrate cooler rocks, the silica will precipitate as quartz in fissures, forming thin white seams as well as large veins which may extend over many kilometres (Bons, 2001; Wangen and Munz, 2004; Pati et al, 2007). In most cases, the quartz in these veins will be massive, but they may also contain well-formed quartz crystals. Phyllites and schists often contain thin lenticular or regular veins of so-called "segregation quartz" (Vinx, 2013) that run parallel to the bedding and are the result of local transport of silica during metamorphosis (Chapman, 1950; Sawyer and Robin, 1986). Silica-rich fluids are also driven out of solidifying magma bodies. When these hot brines enter cooler rocks, the solution gets oversaturated in silica, and quartz forms.

Along with the silica, metals are also transported with the brines and precipitate in the veins as sometimes valuable ore minerals. The association of gold and quartz veins is a well-known example. Quartz is the most common "gangue mineral" in ore deposits.

Quartz Crystals
Quartz crystals typically grow in fluids at elevated temperatures between 150°C and 600°C, but they also grow at ambient conditions (Mackenzie and Gees, 1971; Ries and Menckhoff, 2008).

Quartz is best known for the beautiful crystals it forms in all sorts of cavities and fissures. The greatest variety of shapes and colours of quartz crystals comes from hydrothermal ore veins and deposits, reflecting large differences in growth conditions in these environments (chemistry, temperature, pressure). Splendid, large crystals grow from ascending hot brines in large fissures, from residual silica-rich fluids in cavities in pegmatites and from locally mobilised silica in Alpine-type fissures. An economically important source of amethyst for the lapidary industry are cavities of volcanic rocks. Small, but well-formed quartz crystals are found in septarian nodules and in dissolution pockets in limestones.

Well-formed quartz crystals that are fully embedded in sedimentary rocks and grew during diagenesis (so-called authigenic quartz crystals) are occasionally found in limestones, marls, and evaporites (e.g. Rykart, 1984).

Euhedral quartz crystals that are embedded in igneous rocks are uncommon. Quartz is among the last minerals that form during the solidification of a magma, and because the crystals fill the residual space between the older crystals of other minerals, they are usually irregular. Euhedral, stubby bipyramidal quartz crystals are occasionally found in rhyolites. These are usually paramorphs after beta-quartz with hexagonal symmetry; quartz crystals whose trigonal habit shows that they grew as alpha-quartz are very rare in volcanic rocks (e.g. Flick and Weissenbach, 1978). Only rarely are euhedral quartz crystals seen embedded in metamorphic rocks (Kenngott, 1854; Tschermak, 1874; Heddle, 1901).

Identification

In most cases, quartz is easy to identify by its combination of the following properties:
- hardness (easily scratches glass, also harder than steel)
- glass-like lustre
- poor to indistinct cleavage
- conchoidal fracture in crystals, in massive specimens, the fracture often looks irregular to the naked eye, but is still conchoidal at high magnification.

Note that in macrocrystalline quartz the fracture surfaces have a vitreous to resinous lustre, whereas in cryptocrystalline quartz (chalcedony), fractured surfaces are dull.

Crystals are very common and their usually six-sided shape and six-sided pyramidal tips are well-known. Intergrown crystals without tips can often be recognised by the presence of the characteristic striation on the prism faces.

Quartz as a rock-forming mineral, in particular as irregular grains in the matrix, occasionally poses problems and may require additional means of identification. It may be confused with cordierite (pleochroic, tendency to alteration) and nepheline (lower hardness, geological environment incompatible with quartz).

In thin sections, macrocrystalline quartz appears clear and homogeneous, with blue-grey to white or bright yellow interference colours and a low relief. Quartz does not show alterations at grain boundaries. Strained quartz grains from metamorphic rocks show a so-called "undulatory extinction" (Blatt and Christie, 1963).

ID Requirements on Mindat

Quartz is one of the few minerals on Mindat where "visual identification" may be accepted as a method of identification for new locality entries and photos of well-formed crystals. In other cases, at least hardness should be checked, too.
For quartz as a rock-forming mineral, visual identification is often insufficient.

Handling Quartz

Quartz normally does not require special attention when handled or stored. At ambient conditions, quartz is chemically almost inert, so it does not suffer from the problems seen in many other minerals. Crystals do not disintegrate or crumble, they do not oxidise or dissolve easily in water, and they don't mind being touched. The only problem for the collector is dust, which will find and cover your crystals, no matter what you do.
Quartz crystals that contain large fluid or gas inclusions may crack when heated - skeleton quartz is the most sensitive variety in this respect - but most quartz specimens can take some heat, like cleaning in warm water, without being damaged.
Quartz is hard but quite brittle, and with some effort, one can damage a crystal even with much softer things. The edges of the crystals are very often slightly damaged because the crystals were not kept separate from each other.

Colored quartz varieties can pale in sunlight; the most sensitive variety is euhedral rose quartz/pink quartz, which should be kept in the dark. Amethyst, smoky quartz and natural citrine will also pale, but it takes a very long time.

Mild ultrasonic cleaning is usually not a problem as long as the crystals are not internally cracked, but some varieties may be damaged, in particular, amethyst (due to its polysynthetic Brazil law twinning) and skeleton quartz with liquid and gas inclusions.

Rock Currier wrote a Mindat article on cleaning quartz that is worthwhile reading: http://www.mindat.org/article.php/403/Cleaning+Quartz

When cutting, grinding and polishing specimens, keep in mind that quartz dust will cause silicosis (for a review, see Goldsmith, 1994); do not cut or grind dry and wear an appropriate dust mask.

Quartz bears, on average, 10 ppmw (5 ppmw median) of water. Crystals rich in OH defects may bear as much as 250 ppmw (maximum).