Onyx

Description

Onyx is a parallel-banded variety of chalcedony, which is a microcrystalline or cryptocrystalline form of quartz. It is characterized by its straight, parallel bands of alternating colors, most commonly black and white, but can also include shades of brown, red, and green. The term 'onyx' is often colloquially applied to solid black chalcedony, but scientifically, true onyx refers to the banded variety. Its fine-grained structure gives it a smooth, waxy to vitreous luster when polished.

How to Identify

Color

Typically black and white, or brown and white bands. Can also be solid black (often commercially called 'black onyx' but scientifically solid black chalcedony). Other colors like red, green, or blue can occur due to impurities or artificial dyeing.

Luster

Waxy to vitreous (glassy) when polished; dull to waxy on unpolished surfaces.

Texture

Smooth, microcrystalline, conchoidal fracture surfaces. Feels cool to the touch.

Crystal Form

Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Forms in botryoidal, mammillary, or stalactitic masses, often filling cavities.

Cleavage

None. Exhibits conchoidal fracture.

Geological Environment

Found in the cavities of volcanic rocks (e.g., amygdules in basalt, geodes), hydrothermal veins, and sometimes in sedimentary deposits where silica-rich waters have permeated and deposited chalcedony.

Key Facts

• Hardness: 6.5-7 on the Mohs scale
• Specific Gravity: 2.58-2.64 g/cm³
• Crystal System: Trigonal (cryptocrystalline aggregates)
• Color: Black, white, brown, red, green; often banded
• Luster: Waxy to vitreous
• Transparency: Translucent to opaque
• Fracture: Conchoidal
• Cleavage: None
• Composition: Silicon dioxide (SiO2)

Quick Check

• Color: Banded (typically black/white, brown/white) or solid black
• Luster: Waxy to vitreous
• Streak: White

Physical Characteristics

• Crystal Habit: Cryptocrystalline aggregates, typically massive, botryoidal, mammillary, or stalactitic, filling cavities. Individual crystals are microscopic.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing smooth, shell-like breaks.
• Tenacity: Brittle
• Luster Type: Waxy to vitreous

Formation

Onyx is a cryptocrystalline variety of quartz (chalcedony) that forms in the low-temperature, hydrothermal deposition of silica-rich solutions. It typically precipitates in cavities of volcanic rocks (e.g., basalts, rhyolites) or in sedimentary environments. The characteristic banding results from rhythmic deposition of silica, often with varying amounts of impurities (such as iron oxides, carbonaceous matter) or differences in the porosity of the deposited layers, leading to alternating colors and translucency. The bands are typically parallel and straight, distinguishing it from agate, where bands are often curved or concentric.

Usage

Onyx has been used since antiquity for carving cameos, intaglios, and other ornamental objects. Its distinct banding makes it suitable for creating contrasting layers in carved pieces. Modern uses include jewelry (cabochons, beads), decorative items, architectural elements (tiles, countertops, wall panels), and sometimes as a material for small sculptures. Black onyx is particularly popular in jewelry.

Age Distribution

Onyx forms in various geological settings and is not restricted to a specific geological age. It can be found in deposits ranging from ancient to relatively recent.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates (Tectosilicates)

Group

Quartz Group (Chalcedony Subgroup)

Crystal System

Trigonal (as individual quartz crystals within the cryptocrystalline aggregate)

Chemical Formula

SiO2

Composition

Silicon dioxide, often with trace impurities (e.g., iron oxides, carbonaceous matter) that contribute to color and banding.

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