African Turquoise

Sedimentary (Jasper variety)

Jasper (often treated or dyed)

Also known as: African Jasper, African Opal Jasper

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Description

African Turquoise is a trade name for a mottled green to blue-green jasper, often treated or dyed to enhance its color and mimic the appearance of genuine turquoise. It is not true turquoise (a hydrous phosphate of copper and aluminum) but rather a variety of chalcedony (microcrystalline quartz). The stone typically exhibits a matrix-like pattern with dark inclusions, which are characteristic of jasper. Its appeal lies in its earthy tones and speckled appearance, making it a popular material for various decorative and jewelry applications.

How to Identify

Color: Typically mottled green to blue-green, often with dark brown, black, or reddish-brown matrix-like inclusions. The color is frequently enhanced by dyeing.
Luster: Dull to vitreous (glassy) on fractured surfaces, often waxy or resinous when polished.
Texture: Fine-grained, dense, and opaque. Can feel smooth when polished.
Crystal Form: Massive, cryptocrystalline aggregates; individual crystals are microscopic and not visible to the naked eye.
Cleavage: None, due to its cryptocrystalline nature.
Geological Environment: Jasper deposits are found in various geological settings, including volcanic rocks, sedimentary beds, and hydrothermal veins. The specific jasper used for 'African Turquoise' is typically sourced from regions in Africa, often associated with volcanic or sedimentary formations where silica-rich fluids have precipitated.

Key Facts

Hardness: 6.5-7 on the Mohs scale (for jasper).
Specific Gravity: 2.58-2.91 (for jasper).
Crystal System: Trigonal (for quartz/chalcedony).
Color: Mottled green, blue-green, often with brown/black/reddish-brown matrix. Color is frequently enhanced.
Luster: Dull to vitreous, waxy, or resinous.
Transparency: Opaque.
Fracture: Conchoidal to uneven.
Cleavage: None.
Composition: Silicon dioxide (SiO2) with various impurities (e.g., iron oxides, clay minerals).

Quick Check

Color: Mottled green to blue-green with dark inclusions (often dyed).
Luster: Dull to waxy/resinous (when polished).
Streak: White (for jasper).

Physical Characteristics

Crystal Habit: Cryptocrystalline, massive, nodular, botryoidal.
Cleavage Type: None.
Fracture Type: Conchoidal to uneven.
Tenacity: Brittle.
Luster Type: Dull, waxy, or resinous.

Formation

Jasper forms as a chemical sedimentary rock or as a hydrothermal precipitate. It typically originates from the precipitation of silica-rich solutions in voids, fissures, or as replacement of other minerals or organic material. The characteristic patterns and colors are due to impurities (e.g., iron oxides, clay minerals) incorporated during formation. The 'African Turquoise' appearance is often achieved through dyeing and stabilization treatments of a specific type of jasper, typically a mottled green or blue-green variety.

Usage

Primarily used in jewelry (beads, cabochons), ornamental carvings, and decorative items. Its attractive appearance and relative affordability make it popular in the craft and fashion industries.

Age Distribution

Varies widely depending on the specific jasper deposit; can range from Precambrian to Cenozoic.

Where to Find

Africa (various countries)

While the name 'African Turquoise' suggests a specific origin, the jasper used can come from various African countries. Specific localities are often not disclosed by suppliers due to trade names and treatment processes. Deposits of jasper are widespread across the continent.

Worldwide Jasper Deposits

Jasper, in general, is found globally in locations such as India, Russia, Indonesia, Egypt, Australia, Brazil, Canada, Kazakhstan, Uruguay, Venezuela, and the United States (e.g., Oregon, Idaho, California, Arizona).

Finding Tips

Distinguish from True Turquoise

True turquoise is a phosphate mineral with a distinct chemical composition and often a waxy luster. 'African Turquoise' is a quartz variety (jasper). A simple scratch test (though not recommended for valuable specimens) would show jasper to be harder than turquoise (Mohs 6.5-7 vs. 5-6). Also, true turquoise typically has a slightly lower specific gravity.

Look for Dyeing Indicators

Examine the stone for uneven color distribution, color concentrations in cracks or fissures, or a 'painted' appearance, which can indicate dyeing. Rubbing with acetone on an inconspicuous area might reveal dye transfer, though this is not always conclusive for stabilized or well-treated material.

Observe Matrix Patterns

The dark, often web-like or speckled matrix in 'African Turquoise' is characteristic of jasper. This matrix is typically composed of iron oxides or other mineral inclusions.

Similar Rocks

Turquoise

CuAl6(PO4)4(OH)8·4H2O

Also known as: Callais

Chrysocolla

(Cu,Al)2H2Si2O5(OH)4·nH2O

Also known as: Gem Silica (when pure and translucent)

Variscite

AlPO4·2H2O

Also known as: Utahlite

Other Jaspers

SiO2 (microcrystalline quartz)

Also known as: Picture Jasper, Ocean Jasper

Scientific Classification

Mineral Class: Silicate (Tectosilicate)
Group: Quartz Group (Chalcedony variety)
Crystal System: Trigonal
Chemical Formula: SiO2 (with impurities)
Composition: Silicon dioxide, often with iron oxides, clay minerals, and other trace elements contributing to color and pattern.

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