Ruby in Zoisite

Metamorphic Rock (specifically a metasomatite or skarn-like rock)

Corundum (Ruby) 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, pink to red ruby (a variety of corundum). The vibrant color contrast between the green zoisite and the red ruby, often accented by the dark amphibole, makes it a highly sought-after ornamental material. The ruby crystals are typically anhedral to subhedral and disseminated throughout the zoisite matrix.

How to Identify

Color: Distinctive combination of opaque green (zoisite), opaque pink to 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 individual crystals of ruby and amphibole within the zoisite matrix.
Crystal Form: Ruby: Hexagonal prisms, often anhedral to subhedral within the matrix. Zoisite: Prismatic, often massive. Amphibole: Prismatic to acicular.
Cleavage: Zoisite: Perfect on {010}. Ruby: No true cleavage, but parting on {0001} and {1011}. Amphibole: Two perfect cleavages at approximately 56 and 124 degrees.
Geological Environment: Metasomatic zones associated with the intrusion of felsic magmas into chromium-rich mafic or ultramafic rocks, under conditions of regional or contact metamorphism.

Key Facts

Hardness: Ruby: 9 (Mohs); Zoisite: 6-6.5 (Mohs); Amphibole: 5-6 (Mohs)
Specific Gravity: Ruby: 3.95-4.10; Zoisite: 3.15-3.37; Amphibole (Pargasite): 3.0-3.5
Crystal System: Ruby: Trigonal; Zoisite: Orthorhombic; Amphibole: Monoclinic
Color: Green, red, black
Luster: Vitreous to pearly, adamantine
Transparency: Opaque to translucent (individual minerals can be transparent in thin section)
Fracture: Ruby: Uneven to conchoidal; Zoisite: Uneven; Amphibole: Uneven to subconchoidal
Cleavage: Ruby: No true cleavage, parting on {0001} and {1011}; Zoisite: Perfect on {010}; Amphibole: Perfect on {110} at 56 and 124 degrees
Composition: Mixture of Corundum (Al2O3), Zoisite (Ca2Al3(SiO4)3(OH)), and Amphibole (e.g., Pargasite: NaCa2(Mg,Fe)4Al(Al2Si6O22)(OH)2)

Quick Check

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

Physical Characteristics

Crystal Habit: Ruby: Tabular, prismatic, granular; Zoisite: Prismatic, massive; Amphibole: Prismatic, acicular, massive
Cleavage Type: Ruby: Parting; Zoisite: Perfect; Amphibole: Perfect
Fracture Type: Uneven to conchoidal
Tenacity: Brittle
Luster Type: Vitreous to pearly, adamantine

Formation

Ruby in Zoisite forms through regional or contact metamorphism and metasomatism of chromium-rich mafic or ultramafic rocks (like serpentinites or gabbros) that have been intruded by felsic igneous rocks (like pegmatites or granites). The chromium from the mafic/ultramafic rocks reacts with aluminum and oxygen under high temperature and pressure to form ruby (Cr-rich corundum), while the calcium, aluminum, and silicon from the host rock and metasomatic fluids form zoisite. The black amphibole (often pargasite or hornblende) forms from other available elements.

Usage

Primarily used as an ornamental stone, for carvings, cabochons, spheres, and other decorative items. It is also popular in lapidary arts and as a collector's specimen. The ruby inclusions are generally opaque and not of gem quality for faceting.

Age Distribution

Precambrian to Cenozoic, depending on the specific geological setting of metamorphism and metasomatism.

Where to Find

Tanzania

The primary and most famous source, particularly the Longido mining district near Mount Kilimanjaro. This is where the material was first discovered and named Anyolite.

India

Some occurrences have been reported, though less prominent than Tanzanian material.

Kenya

Similar geological settings to Tanzania, with some reported finds.

Norway

Minor occurrences of zoisite with corundum have been noted.

Finding Tips

Geological Context

Look for areas with evidence of metasomatism where felsic intrusions have interacted with chromium-rich mafic or ultramafic rocks.

Color Recognition

The distinctive green, red, and black coloration is a key identifier. The red ruby stands out against the green zoisite.

Hardness Test

Ruby (9 on Mohs) will scratch zoisite (6-6.5 on Mohs) and amphibole (5-6 on Mohs). This can help confirm the presence of ruby.

Associated Minerals

Often found with other metamorphic minerals characteristic of the host rock and metasomatic alteration.

Similar Rocks

Ruby in Fuchsite

Corundum (Ruby) in Fuchsite

Also known as: Ruby Fuchsite

Ruby in Kyanite

Corundum (Ruby) in Kyanite

Also known as: Ruby Kyanite

Epidote

Ca2(Al,Fe)3(SiO4)3(OH)

Also known as: Pistacite

Scientific Classification

Mineral Class: Oxide (Corundum), Sorosilicate (Zoisite), Inosilicate (Amphibole)
Group: Corundum Group, Epidote Group, Amphibole Group
Crystal System: Trigonal (Corundum), Orthorhombic (Zoisite), Monoclinic (Amphibole)
Chemical Formula: Mixture of Al2O3 (Corundum), Ca2Al3(SiO4)3(OH) (Zoisite), and complex silicate for Amphibole
Composition: Aluminum oxide (Corundum), Calcium Aluminum Silicate Hydroxide (Zoisite), and complex Calcium Magnesium Iron Aluminum Silicate Hydroxide (Amphibole)

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