How to Identify Quartz — Hardness 7, All Varieties & Collecting Guide

Quick Facts

Property	Value
Mineral class	Silicate — tectosilicate framework structure
Chemical formula	SiO₂ (silicon dioxide)
Crystal system	Trigonal (hexagonal appearance)
Mohs hardness	7 — the reference standard for Mohs 7
Luster	Vitreous (glassy) on fresh surfaces; waxy in microcrystalline forms
Color	Colorless when pure; purple (amethyst), yellow (citrine), pink (rose), brown-black (smoky), white (milky)
Streak	White to colorless regardless of surface color
Cleavage	None — breaks with conchoidal fracture in all directions
Fracture	Conchoidal — smooth, curved, shell-like
Transparency	Transparent to translucent (macrocrystalline); opaque (microcrystalline)
Specific gravity	2.65
Crystal form	Hexagonal prisms with pyramidal terminations; horizontal striations on prism faces
Piezoelectric	Yes — generates electric charge under pressure
Acid reaction	None — SiO₂ is insoluble in weak acids
Primary occurrence	Granite, pegmatite, hydrothermal veins, sandstone, quartzite

What Is Quartz?

Quartz is the most abundant mineral in Earth's continental crust — and the most important mineral in rockhounding. Silicon dioxide (SiO₂) in its most fundamental form, quartz crystallizes in a three-dimensional framework where each silicon atom bonds to four oxygen atoms in a tetrahedral arrangement, and each oxygen bridges two silicon atoms. This structure creates a mineral of exceptional chemical stability: quartz resists weathering that destroys almost every other common mineral, which is why it survives and accumulates in sand, sandstone, and beach deposits while feldspars break down to clay and mafic minerals rust to iron oxide.

Quartz's properties are consistent and measurable: Mohs hardness exactly 7 (it is the reference mineral for Mohs 7 on the original 1812 scale), no cleavage in any direction, conchoidal fracture, vitreous luster, and chemical inertness to all common acids. Pure quartz is colorless. But quartz is extraordinarily versatile in appearance: trace elements and irradiation effects produce the full spectrum of colored varieties — purple amethyst, yellow citrine, pink rose quartz, brown to black smoky quartz — while changes in crystallization scale produce the microcrystalline forms that collectors know as chalcedony, jasper, agate, chert, and flint. All of these are quartz by chemistry; all have hardness 7; none have cleavage.

For rockhounds, quartz is simultaneously the most common mineral encountered and the source of some of the hobby's most prized specimens. Crystal-clear quartz clusters from Arkansas command hundreds of dollars. Deep purple amethyst geodes from Brazil fill entire gem show tables. Precisely banded agate nodules are lapidary masterpieces. The quartz family — broadly defined — encompasses more collectible varieties than any other mineral family.

The Chemical Formula of Quartz — SiO₂ Explained

Quartz's chemical formula is SiO₂ — one silicon atom bonded to two oxygen atoms. This is among the simplest formulas in mineralogy, yet it describes one of the most structurally complex minerals. Each silicon is actually bonded to four oxygens (not two) — the formula SiO₂ reflects the fact that each oxygen is shared between two silicon atoms, so each silicon effectively has 4 × (½ oxygen) = 2 oxygens in the formula count.

Why color doesn't change the formula

All quartz varieties — clear, purple, pink, yellow, brown, black — have the same SiO₂ formula. Color comes from trace-level impurities and structural effects, not from bulk composition changes:

Variety	Color	Cause	Still SiO₂?
Rock crystal	Colorless, clear	Pure SiO₂, no impurities	Yes
Amethyst	Purple	Fe³⁺ ions + natural irradiation	Yes
Citrine	Yellow-orange	Fe³⁺ (different configuration or heat treatment)	Yes
Rose quartz	Pink to rose	Ti, Mn, or Fe trace ions; fibrous inclusions	Yes
Smoky quartz	Brown to black	Aluminum impurities + natural radiation	Yes
Milky quartz	White, opaque	Microscopic fluid inclusions scattering light	Yes
Rutilated quartz	Clear with gold/red needles	Rutile (TiO₂) inclusions — separate mineral	Yes (matrix)

The silicon-oxygen tetrahedron

The structural unit of quartz — and all silicate minerals — is the silicon-oxygen tetrahedron: one silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron. In quartz, these tetrahedra link corner-to-corner in three dimensions without any aluminum, iron, or other atoms interrupting the pattern. This pure SiO₂ framework is what gives quartz its extraordinary hardness, inertness, and structural stability.

How to Identify Quartz in the Field

Quartz is one of the most recognizable minerals once you know the key properties. The challenge is distinguishing it from calcite (which can look very similar), glass (which is essentially man-made amorphous SiO₂), and feldspar (the other dominant mineral in many rocks).

What you need:

Steel nail or pocket knife (hardness ~5.5)
Glass plate or glass bottle (hardness ~5.5)
Unglazed porcelain streak plate
Dropper bottle of dilute HCl or white vinegar
10x hand lens or loupe

Step 1 — Test the hardness: Mohs 7

Drag a corner of the specimen firmly across glass. Quartz is Mohs 7 — it scratches glass easily and clearly. The groove left on the glass should be clean and permanent (not a smear that wipes away).

Quartz is the Mohs scale standard for 7 — meaning the hardness test is essentially testing "is this as hard as quartz?" If it scratches glass as easily as a known quartz crystal, it is Mohs 7. Now try to scratch the specimen with a steel nail (Mohs 5.5) — the nail cannot mark quartz.

Material	Can It Scratch Quartz?
Fingernail (Mohs 2.5)	No
Copper penny (Mohs 3.5)	No
Iron nail (Mohs 4)	No
Steel knife/nail (Mohs 5.5)	No
Glass (Mohs 5.5)	No
Quartz on quartz	Yes — equal hardness
Topaz (Mohs 8)	Yes
Corundum/diamond	Yes

Step 2 — Check for cleavage: quartz has none

Examine any broken surface. Quartz has no cleavage — it breaks with smooth, curved, conchoidal fracture regardless of direction. There are no preferred splitting planes controlled by crystal structure.

This separates quartz cleanly from its two most common imposters: calcite has perfect 3-direction rhombohedral cleavage — it always breaks into rhombohedra with flat, slightly pearly faces at ~75° to each other. Feldspar has two cleavage directions at ~90° — breaks produce flat rectangular faces. Fluorite has four perfect cleavage directions — it breaks into octahedra.

If the mineral shows flat, parallel surfaces on any broken face, it has cleavage — it is not quartz. If all breaks are curved and shell-like, it is quartz-family.

Step 3 — The acid test: quartz is inert

Drop dilute HCl or vinegar on the surface. Quartz shows no reaction — no fizzing, no bubbling, no change.

This single test eliminates all carbonate minerals: calcite fizzes immediately and vigorously; dolomite fizzes slowly when cold (vigorous when powdered); aragonite fizzes like calcite.

Even white, transparent, glassy calcite — which visually resembles quartz closely — reveals itself instantly with one drop of acid. A mineral that does not fizz in acid AND scratches glass AND has no cleavage is almost certainly quartz.

Step 4 — Check the streak

Drag the specimen across a porcelain streak plate. Quartz leaves a white to colorless streak regardless of its surface color. Purple amethyst, yellow citrine, and brown smoky quartz all leave white streaks — the color is a surface/lattice phenomenon, not a bulk compositional feature.

Hematite (iron oxide) leaves a red-brown streak. Magnetite leaves a black streak. Malachite leaves a green streak. If a mineral that looks like quartz leaves a colored streak, it is not quartz.

Step 5 — Examine the crystal form and luster

Well-formed quartz crystals show a hexagonal (six-sided) prism capped by a six-faced pyramid (rhombohedra). The prism faces typically show horizontal striations — fine parallel growth lines running perpendicular to the long axis of the crystal. These striations are present on the prism faces but absent from the pyramid faces — this asymmetry is diagnostic.

Luster on fresh surfaces is vitreous — glassy, bright, and clean. In microcrystalline varieties (chalcedony, jasper, agate), luster is waxy to dull — slightly subdued compared to crystalline quartz.

Step 6 — Cross-reference

Combine everything you've observed — hardness, cleavage, acid reaction, streak, and crystal form — to confirm the identification and rule out the closest look-alikes.

An infographic showing the step-by-step means to identifying quartz

Observation	Conclusion
Mohs 7 + no cleavage + no acid fizz + white streak + conchoidal fracture	Quartz confirmed
Mohs 3 + fizzes in acid + rhombohedral cleavage	Calcite — not quartz
Mohs 6 + two-direction cleavage at ~90°	Feldspar — not quartz
Mohs 5.5 + scratched by quartz + may have gas bubbles	Glass — not quartz
Mohs 8 + perfect basal cleavage	Topaz — not quartz
Mohs 4 + four-direction cleavage + often vivid color	Fluorite — not quartz
Mohs 7 but waxy (not glassy) luster + opaque	Microcrystalline quartz (chert, jasper, agate)

Quartz vs. Common Look-Alikes

Mineral / Rock	Luster / Crystal Form	Hardness	Cleavage	Acid Test	Key Distinction
Quartz	Vitreous; hexagonal prism + pyramid	7	None	No fizz	No cleavage; scratches glass; no acid reaction
Calcite	Vitreous to pearly; rhombohedra, scalenohedra	3	Perfect 3-dir. rhombohedral	Vigorous fizz	Fizzes; scratched by nail; perfect cleavage
Feldspar	Vitreous; blocky, striated (plagioclase)	6	2-direction, ~90°	No fizz	2-direction cleavage; slightly softer than quartz
Fluorite	Vitreous; cubes, octahedra	4	Perfect 4-direction octahedral	No fizz	Soft; 4-direction cleavage; vivid color
Topaz	Vitreous; orthorhombic prisms	8	Perfect basal cleavage	No fizz	Harder than quartz; basal cleavage; different form
Glass (man-made)	Vitreous; amorphous — no crystal form	5.5	None	No fizz	Scratched by quartz; may have gas bubbles
Chert / Flint	Waxy to dull; microcrystalline — no visible crystals	7	None	No fizz	Same composition; different microstructure; waxy luster
Gypsum	Pearly to vitreous; plate-like; selenite variety	2	Perfect in 3 directions	No fizz	Very soft — scratched by fingernail

Physical Properties

The Mohs 7 Standard

Quartz is not just Mohs 7 — it IS the definition of Mohs 7. When Friedrich Mohs constructed his 10-mineral hardness scale in 1812, he chose quartz as his Mohs 7 reference mineral. Anything that scratches quartz is above 7; anything quartz scratches is below 7. This makes quartz uniquely useful as a portable field hardness standard — every rockhound who carries a quartz crystal in their kit has a precise Mohs 7 calibration tool.

Piezoelectricity

Quartz is piezoelectric — it generates an electric charge when mechanically deformed. This property, discovered in 1880 by Pierre and Jacques Curie, has made quartz one of the most technologically important minerals in the world:

Oscillators: quartz crystals vibrate at precise, stable frequencies when electrically stimulated. This precision made quartz oscillators the standard for clocks, radios, computers, and GPS systems. Every smartphone contains quartz oscillators.
Transducers: ultrasound medical imaging uses quartz transducers. Sonar systems use them. Microphones and speakers use piezoelectric elements.
Pressure sensors: quartz pressure sensors operate in environments where temperature stability is critical.

The piezoelectric property requires a specific crystal orientation — industrial quartz must be precisely cut at defined angles to the crystal axes. The Arkansas quartz industry grew specifically because of the exceptional quality and size of Arkansas crystals for electronic applications.

Crystallography

Quartz crystal form: trigonal system (class 32 — chiral, meaning left-handed and right-handed quartz crystals exist). The prism has six faces; the termination has six faces (three main rhombohedra and three minor rhombohedra alternating). The horizontal striations on prism faces are a growth artifact — the crystal grew in an oscillating pattern, with alternating faster and slower growth rates leaving subtle ridges.

Left-handed vs. right-handed quartz: quartz comes in mirror-image forms (enantiomorphs) — left-handed and right-handed crystals are identical in most properties but rotate polarized light in opposite directions. The difference is visible in small trapezoidal faces at the junction of prism and pyramid faces that appear on opposite sides of the crystal in L and R forms.

Formation and Geology

Where quartz forms

Quartz crystallizes in four main geological environments:

1. Igneous rocks: quartz crystallizes as a primary mineral in granite, granodiorite, and rhyolite — the felsic igneous rocks where silica content exceeds about 65% SiO₂. As magma cools, quartz is typically the last major mineral to crystallize (after feldspar and mica), filling remaining spaces between earlier crystals. This is why quartz in granite is often anhedral (no well-developed crystal form) — it had to fill whatever space was left.

2. Pegmatites: the late-stage, volatile-rich fluids from crystallizing granite produce pegmatites — exceptionally coarse-grained rocks where crystals can reach meter scale. Quartz in pegmatites often forms giant, perfectly terminated crystals in miarolitic cavities. This is the origin of the world's finest clear quartz crystals and the host environment for amethyst, rose quartz, and smoky quartz in many localities.

3. Hydrothermal veins: hot mineral-rich water circulating through fractures in rock deposits quartz as temperature and pressure drop. Quartz veins cut through almost every rock type on Earth and are frequently associated with metal ore deposits — the "quartz reef" gold veins of Australia, the gold-quartz veins of the California Mother Lode, and countless other ore systems. Hydrothermal quartz is often milky (from fluid inclusions) and may contain gold, sulfide minerals, or other ore minerals.

4. Diagenesis and metamorphism: during burial, quartz can dissolve and reprecipitate as overgrowths on existing quartz grains (silica cement in sandstone) or recrystallize into microcrystalline chert. During metamorphism, quartz recrystallizes extensively — quartzite forms when quartz-rich sandstone is metamorphosed, and quartz veins form throughout metamorphic terranes from fluid migration.

Why quartz survives weathering

The chemical stability of SiO₂ is exceptional. Feldspars (KAlSi₃O₈, NaAlSi₃O₈) break down to clay minerals during chemical weathering — the aluminum and alkali ions are mobilized by slightly acidic water. Mafic minerals (pyroxene, olivine, amphibole) break down to iron oxides, clay, and dissolved ions. Quartz resists all of these reactions because the Si-O bond is extremely strong and SiO₂ has very low solubility in water at normal surface temperatures.

The result: in any sediment or sedimentary rock, the quartz content increases as other minerals are destroyed. Young sediments close to their source contain quartz and feldspar together. Mature sediment that has been transported and reworked for long distances and long times is predominantly quartz. This is why beach sand and sandstone are mostly quartz — they represent the ultimate weathering product of continental rocks.

The Quartz Family — Complete Variety Guide

Quartz occurs in more collectible forms than any other mineral. The varieties divide into two groups based on crystal size: macrocrystalline (individual crystals visible to the naked eye) and microcrystalline (cryptocrystalline — crystals too fine to see without a microscope). All have the same SiO₂ formula and Mohs 7 hardness.

Macrocrystalline Varieties — Individual Crystals Visible

Rock Crystal (Clear Quartz)

Color: colorless, water-clear. Cause: absence of impurities; pure SiO₂. Luster: vitreous — glassy and bright. Best localities: Arkansas (world's finest gem-quality crystals), Brazil, Madagascar.

Rock crystal is quartz in its purest form — water-clear, colorless, glassy. Quality crystals from Arkansas's Ouachita Mountains are among the finest gem quartz in the world. The piezoelectric property of quartz (it generates an electric charge when mechanically compressed) was discovered and exploited commercially using clear rock crystal before synthetic quartz was available. Quartz oscillators using rock crystal's precise vibrational frequency were essential for early radio communications and precision timekeeping.

Collector value: small crystals: $1–10. Quality clusters: $20–200+. Museum specimens from Arkansas: $100–$2,000+.

Amethyst

Color: light lavender to deep purple. Cause: Fe³⁺ ions in the crystal lattice combined with natural irradiation. Best localities: Brazil (largest source), Uruguay, Zambia, Mexico (Guerrero), Arizona (Four Peaks), Thunder Bay, Ontario.

Amethyst is the most valued variety of quartz and one of the most popular collector gemstones. The purple color ranges from pale lavender to deep violet; the finest material ("Siberian" color standard) is a vivid reddish-purple. Brazilian amethyst — often in massive geodes that are sawn open to reveal sparkling crystal linings — is the most common commercial material. Arizona's Four Peaks mine produces a distinctive reddish-purple amethyst of exceptional saturation. Thunder Bay, Ontario produces dark amethyst with hematite inclusions.

Heat treatment converts amethyst to citrine at approximately 470–750°C, which is how most commercial citrine is produced.

Collector value: common small crystals: $1–10. Quality clusters: $20–300. Fine gemstone: $5–30 per carat.

Citrine

Color: yellow to orange-yellow to brownish-orange. Cause: Fe³⁺ in specific lattice positions; often heat-treated from amethyst. Best localities: Brazil, Bolivia; most commercial citrine is heat-treated amethyst.

Natural citrine is rare — pale yellow and found in limited quantities. Most commercial "citrine" (including the vivid orange-yellow "Madeira" and "Spanish citrine" varieties) is heat-treated amethyst from Brazil, where the purple iron-color center converts to yellow under heat. Ametrine — half amethyst, half citrine in a single crystal — occurs naturally in Bolivia's Anahi mine.

A key identification note: natural citrine is typically pale, uniform yellow throughout; heat-treated amethyst often shows concentrated color at the crystal tips with a more brownish or orangey tone, and may show a milky or whitish base. See our how to spot fake gems guide for the full citrine authentication guide.

Collector value: pale natural citrine: $5–30 per carat. Heat-treated material: $2–15 per carat.

Rose Quartz

Color: pale pink to deep rose. Cause: complex — possibly titanium, manganese, or iron ions; also microscopically fine fibrous inclusions of a titanium silicate. Best localities: South Dakota (Custer area, Black Hills), Brazil, Madagascar.

Rose quartz is typically massive (no visible crystal form) with a translucent, milky-pink appearance. Genuine crystal-form rose quartz (showing hexagonal faces) is rare and more valuable than the common massive form. The Black Hills of South Dakota produce significant rose quartz from pegmatites in the Harney Peak granite. Brazil is the world's primary commercial source.

Some rose quartz shows a six-rayed star (asterism) when cut as a cabochon — star rose quartz — caused by rutile needle inclusions similar to those that produce cat's-eye and star effects in other gems.

Collector value: common massive rose quartz: $1–5 per pound. Quality display pieces: $5–30. Star rose quartz cabochon: $10–80.

Smoky Quartz

Color: light tan to deep brown to near-black. Cause: natural radiation (from surrounding rocks) on aluminum impurities in the crystal lattice; reversible by heat. Best localities: Pikes Peak area, Colorado; Pike's Peak granite; Swiss Alps; Brazil.

Smoky quartz is quartz colored by natural radioactivity — aluminum atoms that have replaced silicon in the lattice are ionized by radiation from surrounding minerals, producing a color center that absorbs light. The process is reversible: strong UV light or gentle heating to ~300°C converts smoky quartz back to colorless. Extremely dark smoky quartz (near-black, sometimes called "morion") indicates particularly high radiation exposure.

Colorado's Pikes Peak area is famous for superb smoky quartz in large, perfect crystals from miarolitic cavities (gas pockets) in the Pikes Peak Granite — the same pockets that produce the area's amazonite.

Collector value: small crystals: $1–10. Quality large crystals: $20–200+. Exceptional specimens: $100–$1,000+.

Rutilated Quartz

Color: clear to smoky with golden, red, or silver needle inclusions. Cause: rutile (TiO₂) crystal inclusions growing inside the quartz. Best localities: Brazil, Madagascar, Norway.

Rutilated quartz contains needles or hair-like inclusions of rutile (titanium dioxide) trapped inside the quartz crystal during growth. The inclusions may be golden-yellow, red-brown, or silver and may form random tangles or perfectly parallel arrangements. Collectors prize specimens with well-oriented, high-contrast inclusions against a clear or smoky quartz background.

Collector value: common pieces: $3–20. Fine display quality: $20–200+. Faceted gems: $20–150 per carat.

Herkimer Diamond

Color: colorless, water-clear; occasionally smoky or with anthraxolite inclusions. Cause: unique double-terminated growth habit from free-floating in Cambrian dolostone cavities. Localities: Herkimer County, New York (only significant locality); similar crystals in other locations.

Herkimer diamonds are naturally double-terminated quartz crystals — pointed at both ends — that grew suspended in cavities within Cambrian dolostone rather than attached to a matrix. Their exceptional clarity and natural double termination make them look like cut diamonds (hence the name), though they are Mohs 7 quartz, not diamond. New York's designated state mineral.

Multiple fee-dig operations in Herkimer County allow hands-on collecting. Finding a large, flawless double-terminated crystal is one of the most satisfying experiences in mineral collecting.

Collector value: small common crystals: $3–15. Large, perfect, clear: $50–500+.

Microcrystalline Varieties — Cryptocrystalline Quartz

All microcrystalline quartz varieties have the same SiO₂ composition and Mohs 7 hardness as macrocrystalline quartz but differ in luster (waxy rather than glassy) and transparency (opaque to translucent rather than transparent).

Chalcedony

Color: pale blue to gray to white; translucent. Luster: waxy. Description: the general term for microcrystalline (cryptocrystalline) quartz that is translucent. Blue chalcedony from Oregon and Namibia is particularly prized. Waxy luster, pale color, and translucency are diagnostic.

Agate

Color: any color — the defining feature is banding. Description: banded chalcedony with alternating color bands that record successive layers of silica deposition in a cavity. Among the most popular lapidary and collector minerals. Lake Superior agates, Montana moss agates, and Brazilian iris agates are among the most prized varieties.

→ Find agate collecting locations on our map

Jasper

Color: red, yellow, brown, green, multi-color — opaque. Description: opaque, iron-oxide-colored chalcedony. The color is caused by hematite and goethite dispersed throughout the microcrystalline silica. Picture jasper, poppy jasper, and brecciated jasper are popular lapidary materials.

Carnelian

Color: orange-red to red-orange; translucent. Description: translucent chalcedony colored by hematite/goethite. Unlike opaque jasper, carnelian transmits light at thin edges. Historic use as seals and signet stones goes back millennia.

Chert and Flint

Color: gray, black, brown, white, red. Description: the most common microcrystalline quartz form — found as nodules in limestone and chalk. The "Is chert quartz?" answer: chemically yes (SiO₂), but technically it has a different microstructure from macrocrystalline quartz.

Tiger's Eye

Color: golden brown with chatoyant shimmer. Description: fibrous quartz that replaced crocidolite asbestos, retaining the fibrous structure that produces chatoyancy (cat's-eye effect). See our dedicated tiger's eye guide.

Aventurine

Color: green (most common), blue, orange. Description: quartzite or dense microcrystalline quartz with sparkly metallic inclusions (fuchsite mica for green; hematite or goethite for orange; dumortierite or blue quartz for blue). The sparkle is called aventurescence.

Is Chert Quartz? — The Complete Comparison

One of the most frequently searched questions about quartz. Here is the complete answer.

Chemically: yes — chert and quartz are both SiO₂. Same formula, same silicon-oxygen bond type, same inertness.

Structurally: different. Macrocrystalline quartz has individual crystals ranging from millimeter-scale to meter-scale, with well-developed crystal faces and optical properties you can observe with a loupe. Chert (and flint) is cryptocrystalline — individual crystals are nanometer-scale, far beyond optical microscope resolution. This size difference in crystal structure produces slightly different physical properties:

Property	Macrocrystalline Quartz	Chert / Flint
Crystal size	Visible to eye (>1mm)	Invisible even microscopically (nanometers)
Luster	Vitreous — glassy, bright	Waxy to dull — subdued
Transparency	Transparent to translucent	Opaque (mostly)
Fracture quality	Good conchoidal	Excellent conchoidal — sharper edges
Common form	Crystals, veins, massive	Nodules in limestone; bedded
Hardness	7	7
Chemical formula	SiO₂	SiO₂

The practical answer: chert is composed of the same material as quartz and will test the same on hardness and acid tests. The difference is structural rather than chemical, and in casual collecting usage, chert is routinely called "silica" or "quartz" without error. The mineralogical distinction matters in technical contexts. See our flint wiki page for the full field-identification guide.

Quartz vs. Quartzite — The Rock vs. Mineral Question

Feature	Quartz (Mineral)	Quartzite (Rock)
What it is	Single mineral (SiO₂)	Rock — metamorphic aggregate of quartz grains
Origin	Crystallizes from magma, fluids, or diagenesis	Sandstone metamorphosed by heat and pressure
Grain structure	Single crystal or massive	Interlocking quartz crystals (no visible grain boundaries)
Fracture behavior	Breaks through crystal (conchoidal)	Breaks through grains and cement equally — no gritty feel
Porosity	None in crystals	Very low — grains fused by metamorphism
Feels like	Glassy on broken surface	Smooth, almost glassy; "sugary" surface
Hardness	7	7+ (pure quartzite)

The rockhound's quick test: does it feel gritty (individual grains surface) or smooth and glassy? Sandstone feels gritty. Quartzite feels smooth. Pure quartz crystals feel glassy on broken surfaces. All three scratch glass; only the feel and grain structure differs. See our quartzite wiki page for more.

Where Rockhounds Find Quartz

→ The full guide — 202 verified locations across all 50 states: Find Quartz Near You — Complete U.S. Collecting Guide

Key localities by variety

Clear Quartz Crystals — Arkansas

The Ouachita Mountains of Garland and Montgomery Counties, Arkansas, produce the world's finest clear quartz crystals. The Mount Ida area has multiple fee-dig mines (Ron Coleman Mining, Twin Creek Crystal Mine, Wegner Crystal Farm) where visitors dig their own crystals from clay-filled veins. Arkansas quartz crystal output supplied the entire U.S. electronics industry during World War II for military radios and sonar.

Access: 🟡 Fee-dig operations: $20–50/day for access, keep what you find.

→ Find Arkansas collecting sites

Amethyst — Four Peaks, Arizona

Arizona's Four Peaks mine, in the Mazatzal Mountains east of Phoenix, produces distinctive reddish-purple amethyst from a high-altitude vein system. The mine is active and not open to casual collecting, but its material is widely available through dealers. Thunder Bay, Ontario (technically Canada) produces dark amethyst with included hematite.

Access: 🔴 Four Peaks: private, no collecting access.

Rose Quartz — Black Hills, South Dakota

The Black Hills pegmatites around Custer, South Dakota, expose rose quartz in large masses in road cuts and outcrops throughout the National Forest. Hand-tool collecting on Black Hills National Forest land is free.

Access: 🟢 Free on Black Hills National Forest land.

→ Find South Dakota collecting sites

Smoky Quartz — Pikes Peak Area, Colorado

The Pikes Peak granite's miarolitic cavities produce exceptional smoky quartz and amazonite in Pike National Forest. The Crystal Peak area east of Lake George is accessible and productive. Free collecting with hand tools on Forest Service land.

Access: 🟢 Free on Pike National Forest land.

→ Find Colorado collecting sites

Herkimer Diamonds — Herkimer County, New York

Multiple fee-dig operations (Herkimer Diamond Mines, Ace of Diamonds Mine, Crystal Grove) allow visitors to dig in Cambrian dolostone for the famous double-terminated crystals.

Access: 🟡 Fee-dig: $20–30/person/day.

→ Find New York collecting sites

Agate and Chalcedony — Widespread

Lake Superior agates (Minnesota, Wisconsin, Michigan shorelines), Montana moss agates (Missouri River Breaks BLM), Oregon coast agates (beach collecting, state law permits reasonable quantities).

Access: 🟢 Largely free on public beaches and BLM land.

→ Find quartz collecting locations on the interactive map

Industrial and Technological Uses of Quartz

Electronics and technology

Quartz's piezoelectric property made it the cornerstone of 20th-century electronics:

Oscillators: every digital clock, radio, computer, and GPS device contains quartz crystal oscillators
Resonators: frequency control for telecommunications
Transducers: ultrasound imaging, sonar, microphones, pressure sensors
Optical applications: UV-transparent quartz optics for UV spectroscopy and UV-germicidal systems

Glass and silicon

Silica glass (fused quartz): extremely high-purity quartz is melted to produce fused silica glass for telescope mirrors, optical fibers, semiconductor photomasks, and UV-transmitting optics
Silicon metal: quartz is reduced by carbon to produce silicon metal, the feedstock for all silicon chips and solar cells
Glass industry: silica sand (primarily quartz) is the primary raw material for container glass, float glass (windows), and fiberglass

Construction and industrial

Hydraulic fracturing (fracking) proppant: high-sphericity quartz sand is pumped into oil and gas wells to hold fractures open
Foundry sand: quartz sand molds are used in metal casting
Abrasives: quartz sandpaper, sandblasting media
Filtration: quartz sand in water filtration systems

Value and Collectibility

Quartz Variety	Typical Rough Value	Gem/Faceted Value
Rock crystal (clear)	$1–20/lb	$5–50/carat (fine quality)
Amethyst	$2–30/lb rough	$5–30/carat
Citrine (natural)	$5–30/lb	$10–50/carat
Rose quartz	$1–10/lb	$2–20/carat
Smoky quartz	$2–20/lb	$5–30/carat
Rutilated quartz	$5–30/piece	$20–150/carat
Herkimer diamond	$5–30/crystal	Up to $500 for exceptional
Agate	$1–20/lb	$5–50/slab
Jasper	$1–10/lb	$2–20/slab
Tiger's eye	$2–15/lb	$5–30/carat
Chalcedony (gem blue)	$10–50/lb	$10–80/carat

Frequently Asked Questions

What is quartz?

Quartz is the most abundant mineral in Earth's continental crust — silicon dioxide (SiO₂) forming in a trigonal crystal structure with consistent properties: Mohs hardness 7 (the Mohs scale standard), no cleavage, conchoidal fracture, vitreous luster, and chemical inertness to weak acids. It occurs in transparent crystals (rock crystal, amethyst, citrine, rose quartz, smoky quartz) and microcrystalline forms (chalcedony, jasper, agate, chert, flint).

What is the hardness of quartz on the Mohs scale?

Quartz hardness is exactly 7 on the Mohs scale — quartz is the reference mineral that defines Mohs 7. It scratches glass (Mohs 5.5) easily and cannot be scratched by a steel nail (Mohs 5.5).

What is the chemical formula of quartz?

Quartz's chemical formula is SiO₂ — silicon dioxide. All quartz varieties (amethyst, citrine, rose quartz, smoky quartz, chalcedony, jasper, agate, chert) have the same SiO₂ formula. Color differences come from trace elements and radiation effects, not from compositional changes.

Is quartz a mineral or a rock?

Quartz is a mineral — a single compound (SiO₂) with defined crystal structure and consistent properties. Quartzite (metamorphic), sandstone (sedimentary), and granite (igneous) are rocks that contain quartz but are aggregates of multiple minerals.

Is chert quartz?

Chert is composed of the same chemical substance as quartz (SiO₂) but has a different internal structure — cryptocrystalline (nanometer-scale crystals) rather than macrocrystalline. Chert has the same hardness (7) and no acid reaction as quartz, but has a waxy rather than glassy luster and is opaque rather than transparent.

What is the difference between quartz and quartzite?

Quartz is a mineral; quartzite is a metamorphic rock composed of interlocking quartz crystals formed when sandstone was heated and pressured. Sandstone feels gritty (individual grain surfaces); quartzite feels smooth and glassy (grains fused together). Both scratch glass at Mohs 7.

Where is quartz found?

Quartz is found virtually everywhere. For collectors: Arkansas (world's finest clear crystals, fee-dig mines), Arizona (amethyst at Four Peaks), South Dakota Black Hills (rose quartz, free on BHNF), Colorado Pikes Peak area (smoky quartz, free on Pike NF), New York Herkimer County (Herkimer diamonds, fee-dig).