Ruby in Zoisite

Metamorphic Rock (specifically a Metasomatite)

Corundum in Zoisite

Also known as: Anyolite

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Description

Ruby in Zoisite, also known as Anyolite, is a striking metamorphic rock composed primarily of green zoisite, black amphibole (often pargasite or hornblende), and opaque red corundum (ruby). The vibrant green of the zoisite provides a strong contrast to the deep red rubies, often with black patches or streaks of amphibole. It is not a single mineral but a rock assemblage.

How to Identify

Color: Distinctive combination of opaque green (zoisite), opaque red (ruby), and black (amphibole).
Luster: Zoisite: Vitreous to pearly; Ruby: Adamantine to vitreous; Amphibole: Vitreous to dull.
Texture: Granular to massive, often with visible ruby crystals embedded within the zoisite matrix.
Crystal Form: Ruby typically occurs as hexagonal prisms or irregular grains. Zoisite is massive or columnar. Amphibole is acicular or massive.
Cleavage: Zoisite: Perfect on {010}; Ruby: No true cleavage, but parting on {0001} and {1011}; Amphibole: Two distinct cleavages at approximately 56 and 124 degrees.
Geological Environment: Metasomatized mafic or ultramafic rocks, often associated with regional metamorphic terrains, particularly in areas with chromium enrichment.

Key Facts

Hardness: Variable, 6-7 for zoisite, 9 for ruby, 5-6 for amphibole (Mohs scale)
Specific Gravity: Variable, 3.1-3.4 for zoisite, 3.9-4.1 for ruby, 3.0-3.4 for amphibole
Crystal System: Zoisite: Orthorhombic; Ruby (Corundum): Trigonal; Amphibole: Monoclinic
Color: Green (zoisite), Red (ruby), Black (amphibole)
Luster: Vitreous to pearly (zoisite), Adamantine to vitreous (ruby), Vitreous to dull (amphibole)
Transparency: Opaque to translucent (zoisite), Opaque (ruby), Opaque (amphibole)
Fracture: Uneven to conchoidal (zoisite), Conchoidal to uneven (ruby), Uneven (amphibole)
Cleavage: Zoisite: Perfect on {010}; Ruby: No true cleavage, parting on {0001} and {1011}; Amphibole: Two distinct cleavages at approximately 56 and 124 degrees
Composition: A rock composed of Zoisite (Ca2Al3(SiO4)3(OH)), Corundum (Al2O3, specifically ruby due to Cr3+ impurities), and Amphibole (complex silicates, e.g., pargasite (NaCa2(Mg4Al)Si6Al2O22(OH)2) or hornblende).

Quick Check

Color: Green, red, and black combination
Luster: Vitreous to pearly (zoisite), adamantine (ruby), vitreous to dull (amphibole)
Streak: White (zoisite), white (ruby), greenish-white (amphibole)

Physical Characteristics

Crystal Habit: Zoisite: Prismatic, columnar, massive; Ruby: Hexagonal prisms, tabular, massive; Amphibole: Prismatic, acicular, massive.
Cleavage Type: Zoisite: Perfect; Ruby: Parting; Amphibole: Distinct.
Fracture Type: Uneven to conchoidal.
Tenacity: Brittle.
Luster Type: Vitreous to pearly (zoisite), Adamantine to vitreous (ruby), Vitreous to dull (amphibole).

Formation

Ruby in Zoisite forms through metasomatism, a process where the chemical composition of a rock is altered by the introduction or removal of chemical components by hydrothermal fluids. In this case, chromium-rich fluids interact with plagioclase-rich rocks (like anorthosite or gabbro) under amphibolite to granulite facies metamorphic conditions. The chromium substitutes for aluminum in the corundum structure, forming ruby, while the plagioclase is altered to zoisite. The black amphibole (pargasite or hornblende) forms from other mafic minerals present.

Usage

Primarily used as an ornamental stone, for carvings, cabochons, spheres, and decorative objects. It is also popular in lapidary arts and as a collector's mineral specimen. Due to its aesthetic appeal, it is sometimes used in jewelry, though the zoisite component is relatively soft.

Age Distribution

Typically Precambrian to Paleozoic, formed during regional metamorphism and metasomatism.

Where to Find

Longido, Tanzania

The primary and most famous source of high-quality Ruby in Zoisite, discovered in 1956. Occurs in the Precambrian Mozambique Belt.

Kenya

Other occurrences have been reported in East Africa, sharing similar geological settings to Tanzania.

India

Some deposits of ruby-bearing zoisite have been found, though often differing slightly in appearance or composition.

Finding Tips

Geological Context

Look for areas with known occurrences of metasomatized mafic or ultramafic rocks within regional metamorphic belts. Associated minerals might include plagioclase, hornblende, and other chromium-bearing minerals.

Visual Identification

The unique color combination of green, red, and black is highly distinctive. Rubies will appear as opaque, often blocky, red crystals within the green zoisite matrix.

Hardness Test

While the overall rock has variable hardness, the ruby component is very hard (9 on Mohs scale), which can be tested on a fresh surface if other minerals are scratched.

Similar Rocks

Fuchsite with Ruby

Corundum in Fuchsite

Also known as: Ruby in Fuchsite

Epidote with Ruby

Corundum in Epidote

Also known as: Ruby in Epidote

Thulite

Manganiferous Zoisite

Also known as: Pink Zoisite

Scientific Classification

Mineral Class: Silicate (Zoisite, Amphibole), Oxide (Corundum)
Group: Epidote Group (Zoisite), Corundum Group (Ruby), Amphibole Supergroup (Amphibole)
Crystal System: Orthorhombic (Zoisite), Trigonal (Ruby), Monoclinic (Amphibole)
Chemical Formula: Mixture of Ca2Al3(SiO4)3(OH) (Zoisite), Al2O3 (Corundum), and complex silicates (Amphibole)
Composition: Calcium aluminum silicate hydroxide (Zoisite), Aluminum oxide (Corundum), Complex calcium magnesium iron aluminum silicates (Amphibole)

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