Onyx

Chemical formula: SiO₂

Onyx is a variety of chalcedony, characterized by parallel layers of contrasting colors, most commonly black and white.

## Characteristics Onyx is a mineral from the quartz group, being a variety of chalcedony. It is distinguished by its layered structure, where individual layers of varying colors are arranged in parallel. The most well-known variety is black and white onyx, but specimens with other color combinations, such as brown and white or red and white, also occur. These layers are often very thin and regular, which gives the mineral its characteristic appearance. The surface of onyx is usually smooth and can take on a high polish when cut. ## Physical Properties Onyx is characterized by a Mohs hardness of 6.5-7, making it relatively scratch-resistant. It has a conchoidal fracture, typical of minerals from the quartz group. The luster of onyx is vitreous or waxy, and its density ranges from 2.6-2.7 g/cm³. This mineral is opaque to translucent, depending on the thickness and purity of the layers. It does not exhibit cleavage. ## Colors and Varieties The most valued and recognizable variety of onyx is black and white onyx, where contrasting layers form distinct bands. There are also varieties with brown, red, green, and even blue colors, often with white or lighter layers. The name "onyx" is also sometimes used in the context of onyx marbles, which are calcareous sedimentary rocks with a similar layered structure but different chemical composition and physical properties. ## History and Name The name "onyx" comes from the Greek word "onyx," meaning fingernail or hoof, which probably refers to its layered structure resembling fingernail layers. This mineral was known and valued in antiquity, used for making jewelry, seals, and inlays. Mentions of onyx appear in many ancient texts, testifying to its long history of use. ## Applications Onyx is widely used in jewelry making for cabochons, beads, carvings, and inlays. Due to its hardness and ability to achieve a high polish, it is a popular ornamental material. It is also used in sculpture for creating small figurines and decorative objects. In architecture, especially onyx marble, it is sometimes used as a cladding and decorative material.

Properties

Mohs hardness: 6.5-7
Luster: Szklisty do woskowego
Streak: Biała
Density: 2.6-2.7
Cleavage: Brak
Fracture: Muszlowy
Transparency: Nieprzezroczysty do przeświecającego
Crystal system: Trygonalny

Diagnostic features

## Identification Onyx can be identified by its characteristic, parallel layered structure, where layers of contrasting colors are clearly visible. A Mohs hardness of 6.5-7 and conchoidal fracture are also helpful in identification. Vitreous or waxy luster is typical for this mineral. ## Distinguishing from Similar Minerals Onyx is sometimes confused with agate, which is also a variety of chalcedony with a layered structure. The main difference is that in agate, the layers are usually concentric or irregular, while in onyx, they are parallel and straight. Onyx marble, although visually similar due to its layered structure, is a carbonate rock and is much softer than onyx, reacting with acids. ## Crystal Forms Onyx does not form macroscopic crystals in the typical sense. It is a cryptocrystalline variety of quartz, meaning its crystalline components are too small to be visible to the naked eye. It occurs as massive aggregates with a layered structure, often in the form of concretions or vein fillings.

Geological environment

## Genesis Onyx forms under low-temperature hydrothermal conditions, often in geodes and rock fissures. It is a product of precipitation from siliceous solutions. This process leads to the formation of layers with different chemical compositions or impurities, resulting in color variations. ## Mineral Associations Onyx often coexists with other varieties of chalcedony, such as agate, jasper, as well as quartz, opal, and calcite. It can be found in volcanic rocks, such as basalts and andesites, where it fills empty spaces. ## Localities Significant deposits of onyx occur in many places around the world. The most important include India, Brazil, Uruguay, Mexico, Argentina, the United States (Arizona, California), as well as countries in the Middle East and North Africa.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens of onyx are characterized by a clear contrast between layers, regularity, and thinness of the bands. A high polish after cutting, absence of cracks and inclusions, and an attractive color combination also increase the value of the specimen. Large, well-colored, and flawless blocks are particularly sought after. ## Popular Localities High-quality specimens often come from India, Brazil, and Uruguay, where chalcedony deposits with excellent properties are found. Mexico is also known for beautiful varieties of onyx, including onyx marbles.

Care and storage

## Cleaning Onyx should be cleaned gently, using a soft cloth or brush and lukewarm water with a small amount of mild soap. After washing, the mineral should be thoroughly rinsed with clean water and dried with a soft cloth to avoid water spots. ## What to Avoid Avoid contact of onyx with strong chemicals, acids, and bases, which can damage its surface or alter its color. It is also not recommended to expose onyx to sudden temperature changes, which can lead to cracks. Prolonged exposure to direct sunlight can cause some varieties of onyx to fade. ## Storage Onyx is best stored separately from other, harder minerals to prevent scratches. An ideal place is a soft pouch, a fabric bag, or a separate compartment in a jewelry box. Displayed specimens should be protected from dust and direct light.