How to identify Quartz varieties and Onyx
Quartz (SiO2) varieties and Onyx (SiO2)
Got a photo? Identify rocks instantly with the Rockby app
Open the appTo correctly identify Quartz varieties and Onyx (Quartz (SiO2) varieties and Onyx (SiO2)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.
Step-by-step identification
- 1
Color
Overall color and any zoning or banding
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.
- 2
Luster
How the surface reflects light: metallic, vitreous, dull
Vitreous (glassy) for all varieties.
- 3
Texture
Grain size and surface feel: coarse, fine, glassy
Smooth to conchoidal fracture surfaces. Beads will be polished smooth.
- 4
Crystal Form
Shape of visible crystals or crystal faces
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.
- 5
Cleavage
How it breaks: flat cleavage planes vs irregular fracture
None (lacks cleavage).
- 6
Geological Environment
The rock formation and setting where it occurs
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)
Physical characteristics
- Crystal Habit: Prismatic, massive, cryptocrystalline (for onyx)
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous
Explore Quartz varieties and Onyx
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.