Quartz
Quartz is silicon dioxide, SiO2, one of the most abundant minerals in Earth's continental crust, crystallizing in the trigonal division of the hexagonal crystal system with a hardness of 7 that makes it a standard reference point on the Mohs scale. Its six-sided prismatic crystals, often terminated by pyramidal points, are among the most recognizable crystal shapes in mineralogy. Quartz occurs in nearly every rock type and geologic setting, from granite pegmatites to sedimentary sandstones to hydrothermal veins.
Quartz properties at a glance
| Type | Quartz Family |
|---|---|
| Chemical formula | SiO2 |
| Mohs hardness | 7 |
| Crystal system | Trigonal |
| Luster | Vitreous, greasy on fracture surfaces |
| Streak | White |
| Cleavage | None (rare parting on rhombohedral planes) |
| Fracture | Conchoidal |
| Specific gravity | 2.65 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Purple (amethyst), Yellow (citrine), Pink (rose), Brown/black (smoky) |
| Rarity | common |
Geology & background
Quartz crystallizes across an extraordinarily wide range of temperatures and pressures, forming directly from cooling granitic magma, precipitating from silica-rich hydrothermal fluids in veins, and growing in cavities called geodes where groundwater slowly deposits silica over long periods. Because quartz resists weathering better than most other rock-forming minerals, it also survives erosion to become the dominant grain in sand and sandstone. Legendary localities include the crystal-clear points of Arkansas's Ouachita Mountains, the massive amethyst geodes of Brazil and Uruguay, and the smoky quartz crystals of Switzerland's alpine clefts.
Rockhounds value quartz for its enormous variety, since trace impurities and radiation exposure produce distinct varieties including purple amethyst, yellow citrine, smoky brown quartz, rose quartz, and milky white quartz, each with its own following among collectors. Because quartz is so hard and chemically stable, well-formed crystal clusters survive for display far better than many softer minerals, and even large specimens remain relatively affordable compared to rarer minerals of similar size. Twinned crystals, phantom inclusions, and rutile needles inside clear quartz add further variety prized by specialist collectors.
How to identify Quartz
Quartz forms hexagonal prisms typically capped with pyramidal terminations, ranging from colorless to milky white, purple, yellow, brown, pink, or black depending on trace impurities and irradiation. Its luster is vitreous, streak is white, and hardness of 7 lets it scratch glass and steel easily, a reliable field test since few common minerals match that hardness. It shows no cleavage, breaking instead with a smooth conchoidal fracture, and often displays natural striations running perpendicular to the crystal's length. Quartz is frequently confused with calcite or topaz, but its hardness readily separates it from the much softer calcite.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Quartz is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Quartz worth?
| Entry-level specimen | $5 |
|---|---|
| Typical collector specimen | $35 |
| Premium specimen | $250+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Display
- Education
- Lapidary work when suitable
Interesting facts
- Quartz crystals vibrate at a precise, stable frequency when an electric current passes through them, a property called piezoelectricity that powers most quartz wristwatches.
- Amethyst and citrine are chemically identical quartz, differing only in trace iron content and the type of natural or artificial irradiation each has experienced.
- Sand on most of the world's beaches is dominated by quartz grains because the mineral resists weathering far longer than nearly any other rock-forming mineral.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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