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This collection refers to several varieties of the mineral Quartz (SiO2) and Onyx, which is a banded form of chalcedony (also SiO2). Amethyst is a purple variety, Smoky Quartz is a translucent gray to brown variety, and Clear Quartz is a colorless and transparent variety. Black Onyx is a chalcedony variety characterized by its uniform black color, often achieved through treatment. All are silicates and share many fundamental physical and chemical properties, differing primarily in color and crystal habit (for onyx).
How to Identify
- Color
- Amethyst: purple to violet. Smoky Quartz: translucent gray to brown, sometimes black. Clear Quartz: colorless, transparent. Black Onyx: opaque black, sometimes with very subtle banding visible under magnification.
- Luster
- Vitreous (glassy) for all varieties.
- Texture
- Smooth to conchoidal fracture surfaces. Beads will be polished smooth.
- Crystal Form
- Quartz varieties typically form hexagonal prisms terminating in hexagonal pyramids. Onyx is cryptocrystalline, so individual crystals are microscopic; macroscopic forms are massive, botryoidal, or banded.
- Cleavage
- None (lacks cleavage).
- Geological Environment
- Hydrothermal veins, pegmatites, igneous and metamorphic rocks, sedimentary geodes and concretions (for quartz). Low-temperature hydrothermal deposits, often in volcanic rocks or as replacement minerals (for onyx).
Key Facts
- Hardness: 7 on the Mohs scale
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal
- Color: Amethyst: purple; Smoky Quartz: gray to brown; Clear Quartz: colorless; Black Onyx: black
- Luster: Vitreous
- Transparency: Transparent to translucent (Amethyst, Smoky Quartz, Clear Quartz); Opaque (Black Onyx)
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Amethyst: purple; Smoky Quartz: gray/brown; Clear Quartz: colorless; Black Onyx: black
- Luster: Vitreous (glassy)
- Streak: White (for all varieties)
Physical Characteristics
- Crystal Habit: Prismatic, massive, cryptocrystalline (for onyx)
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous
Formation
Quartz varieties (Amethyst, Smoky Quartz, Clear Quartz) typically form in igneous, metamorphic, and sedimentary environments. They crystallize from silica-rich hydrothermal solutions in veins and geodes, or from magmatic crystallization in granitic rocks. Smoky Quartz's color is often attributed to natural irradiation of aluminum impurities within the crystal lattice. Amethyst's purple color is due to iron impurities and natural irradiation. Clear Quartz is pure silicon dioxide. Onyx is a cryptocrystalline variety of quartz (chalcedony) that forms in the low-temperature deposition of silica from groundwater, often in cavities or as a replacement mineral, exhibiting parallel banding. Black Onyx is typically uniformly black, often achieved through dyeing of naturally gray onyx or chalcedony.
Usage
Widely used in jewelry, ornamental objects, lapidary arts, and as beads. Clear Quartz is also used in electronics (oscillators, transducers) and optics. Amethyst and Smoky Quartz are popular gemstones. Black Onyx is valued for its uniform dark color and polish.
Age Distribution
Formed throughout geological history, from Precambrian to Cenozoic eras, depending on the specific geological setting.
Where to Find
Brazil
Major source of Amethyst (especially Rio Grande do Sul) and Clear Quartz.
Uruguay
Known for high-quality, deep purple Amethyst.
Madagascar
Produces significant quantities of Amethyst, Smoky Quartz, and Clear Quartz.
United States
Various localities for Clear Quartz (e.g., Arkansas), Smoky Quartz (e.g., Colorado), and Amethyst (e.g., North Carolina, Arizona).
Switzerland
Historically known for fine Smoky Quartz.
India
Source of various chalcedony forms, including onyx.
Mexico
Produces Amethyst and other quartz varieties.
Finding Tips
Hardness Test
All quartz varieties have a Mohs hardness of 7, which can scratch glass. This helps differentiate them from softer minerals.
Specific Gravity
Quartz has a specific gravity of 2.65. This can be used to distinguish it from denser or lighter materials if precise measurements are possible.
Absence of Cleavage
The lack of cleavage and presence of conchoidal fracture are key identifiers for quartz.
Color and Transparency
Observe the characteristic colors and transparency: purple for amethyst, smoky gray/brown for smoky quartz, colorless and transparent for clear quartz, and opaque black for black onyx.
Banding (for Onyx)
While black onyx is typically uniform, some natural onyx may show very subtle parallel banding upon close inspection, distinguishing it from other black minerals like obsidian (which is volcanic glass).
Similar Rocks
Chalcedony
SiO2
Also known as: Agate, Jasper, Carnelian
Tourmaline
Complex borosilicate
Also known as: Schorl, Elbaite
Fluorite
CaF2
Also known as: Fluorspar
Scientific Classification
- Mineral Class
- Silicate
- Group
- Tectosilicate (Quartz Group)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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