Ruby in Zoisite

Metamorphic Rock (specifically a metasomatite)

Ruby (Corundum) and 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 to translucent ruby (corundum). The vibrant green color is due to the presence of chromium in the zoisite, while the ruby's red color is also caused by chromium. The black amphibole provides a stark contrast, enhancing the visual appeal of the rock. It is not a single mineral but a rock assemblage. The ruby crystals are typically anhedral to subhedral and can range from small specks to larger, well-formed crystals embedded within the zoisite matrix.

How to Identify

Color: Distinctive combination of opaque green (zoisite), black (amphibole), and pink to deep red (ruby). The green zoisite can vary in intensity, and the ruby can appear as scattered crystals or larger masses.
Luster: Zoisite: Vitreous to pearly. Ruby: Adamantine to vitreous. Amphibole: Vitreous to dull.
Texture: Granular to massive, often with visible individual crystals of ruby, zoisite, and amphibole. The texture can be uneven due to the varying hardness and crystal habits of the constituent minerals.
Crystal Form: Ruby: Hexagonal prisms, often anhedral to subhedral within the matrix. Zoisite: Prismatic, often massive. Amphibole: Prismatic to acicular.
Cleavage: Ruby: No true cleavage, but parting along basal planes. Zoisite: Perfect cleavage on {010}. Amphibole: Two distinct cleavages at approximately 56 and 124 degrees.
Geological Environment: Metamorphic terrains, particularly in areas of regional metamorphism and metasomatism involving mafic and ultramafic rocks. Often found in association with chromium-rich rocks.

Key Facts

Hardness: Ruby: 9 (Mohs). Zoisite: 6-6.5 (Mohs). Amphibole: 5-6 (Mohs).
Specific Gravity: Ruby: 3.95-4.10. Zoisite: 3.10-3.38. Amphibole: 2.9-3.6.
Crystal System: Ruby: Trigonal. Zoisite: Orthorhombic. Amphibole: Monoclinic.
Color: Green (zoisite), black (amphibole), red/pink (ruby).
Luster: Vitreous to pearly (zoisite), adamantine to vitreous (ruby), vitreous to dull (amphibole).
Transparency: Opaque to translucent.
Fracture: Ruby: Uneven to conchoidal. Zoisite: Uneven. Amphibole: Uneven to subconchoidal.
Cleavage: Ruby: No true cleavage, but basal parting. Zoisite: Perfect on {010}. Amphibole: Two distinct cleavages at ~56° and ~124°.
Composition: A rock composed of Ruby (Al2O3), Zoisite (Ca2Al3(SiO4)3(OH)), and various amphiboles (e.g., Pargasite, Hornblende - complex silicates).

Quick Check

Color: Green, black, and red combination
Luster: Vitreous to pearly (zoisite), adamantine to vitreous (ruby)
Streak: White (for zoisite and ruby)

Physical Characteristics

Crystal Habit: Ruby: Hexagonal prisms, often anhedral to subhedral. Zoisite: Prismatic, massive. Amphibole: Prismatic, acicular.
Cleavage Type: Ruby: Parting. Zoisite: Perfect. 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 regional metamorphism and metasomatism. It typically occurs in mafic and ultramafic rocks that have undergone significant alteration. The ruby (corundum) forms from aluminum-rich protoliths under high-temperature and moderate-to-high pressure conditions, often in desilicated environments. The zoisite, an epidote group mineral, forms from the alteration of plagioclase feldspar in calcium-rich rocks during metamorphism. The black amphibole (often pargasite or hornblende) is also a product of metamorphic processes in these environments. The specific conditions involve the interaction of hydrothermal fluids with chromium-bearing rocks, leading to the crystallization of ruby, zoisite, and amphibole.

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 mineral specimen. Due to its striking color contrast, it is highly valued for aesthetic purposes.

Age Distribution

Precambrian to Cenozoic, depending on the specific geological setting and metamorphic events. The primary deposits in Tanzania are associated with Neoproterozoic metamorphic events.

Where to Find

Longido, Tanzania

The primary and most famous source of high-quality Ruby in Zoisite (Anyolite). Found in the Longido mining area, Arusha Region, Tanzania. The deposits are associated with Neoproterozoic metamorphic rocks.

India

Some occurrences of ruby in zoisite have been reported, though generally not as prolific or well-known as the Tanzanian deposits.

Norway

Occurrences of zoisite with corundum have been noted, though not always with the same distinct ruby-in-zoisite aesthetic as Anyolite.

Finding Tips

Look for Metamorphic Terrains

Focus on areas known for regional metamorphism, especially those with mafic and ultramafic rock formations.

Identify Associated Minerals

Ruby in Zoisite often occurs with other metamorphic minerals. Familiarize yourself with the appearance of zoisite, amphiboles, and corundum in their natural settings.

Observe Color and Texture

The distinctive green, black, and red coloration is a key indicator. Look for granular textures with embedded crystals.

Check for Hardness

Ruby is very hard (9 on Mohs scale), while zoisite is softer (6-6.5). This difference can sometimes be felt or observed on weathered surfaces.

Similar Rocks

Fuchsite with Ruby

Fuchsite (Muscovite variety) and Ruby (Corundum)

Also known as: Ruby in Fuchsite

Epidote with Ruby

Epidote and Ruby (Corundum)

Also known as: Ruby in Epidote

Thulite

Zoisite (Manganese-rich variety)

Also known as: Pink Zoisite

Scientific Classification

Mineral Class: Ruby: Oxide. Zoisite: Sorosilicate. Amphibole: Inosilicate.
Group: Ruby: Corundum Group. Zoisite: Epidote Group. Amphibole: Amphibole Supergroup.
Crystal System: Ruby: Trigonal. Zoisite: Orthorhombic. Amphibole: Monoclinic.
Chemical Formula: Ruby: Al2O3. Zoisite: Ca2Al3(SiO4)3(OH). Amphibole: Variable, e.g., Pargasite: NaCa2(Mg4Al)(Al2Si6)O22(OH)2.
Composition: A rock consisting of aluminum oxide (ruby), calcium aluminum silicate hydroxide (zoisite), and complex calcium magnesium iron aluminum silicates (amphibole).

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