Ruby in Zoisite

Description

Ruby in Zoisite, also known as Anyolite, is a striking metamorphic rock composed primarily of three minerals: green zoisite, opaque red ruby (a variety of corundum), and often black hornblende (or other amphiboles). The vibrant green of the zoisite provides a stark contrast to the deep red of the ruby, making it a visually appealing material. The ruby crystals are typically opaque and can range from small inclusions to larger, well-formed crystals embedded within the zoisite matrix. The black amphibole often appears as small, dark specks or veins, further enhancing the rock's aesthetic appeal. It is not a single mineral but a rock assemblage.

How to Identify

Color

Distinctive combination of vibrant green (zoisite), opaque red (ruby), and often black (hornblende). The green can vary from light to dark, and the red of the ruby can be purplish-red to brownish-red.

Luster

Zoisite: Vitreous to pearly. Ruby: Adamantine to vitreous. Hornblende: Vitreous to dull.

Texture

Typically granular to massive, with visible individual crystals of zoisite, ruby, and hornblende. The ruby crystals are often anhedral to subhedral, embedded within the zoisite matrix.

Crystal Form

Zoisite: Prismatic, often elongated. Ruby: Hexagonal prisms, often tabular or barrel-shaped, but typically anhedral to subhedral in this rock. Hornblende: Prismatic.

Cleavage

Zoisite: Perfect on {010}. Ruby: None (parting on {0001} and {1011} can occur). Hornblende: Two distinct cleavages at approximately 56 and 124 degrees.

Geological Environment

Metamorphic terrains, particularly those associated with regional metamorphism of mafic or ultramafic rocks, often in amphibolite facies conditions, with subsequent metasomatic alteration.

Key Facts

• Hardness: Zoisite: 6-7 (Mohs). Ruby: 9 (Mohs). Hornblende: 5-6 (Mohs). The overall rock hardness will vary depending on the proportions of its constituent minerals.
• Specific Gravity: 3.2-3.6 (variable, depending on the proportions of zoisite, ruby, and hornblende)
• Crystal System: Zoisite: Orthorhombic. Ruby (Corundum): Hexagonal (Trigonal). Hornblende: Monoclinic.
• Color: Green, red, black.
• Luster: Vitreous to pearly.
• Transparency: Opaque to translucent (zoisite), Opaque (ruby), Opaque (hornblende).
• Fracture: Zoisite: Uneven to conchoidal. Ruby: Conchoidal to uneven. Hornblende: Uneven.
• Cleavage: Zoisite: Perfect on {010}. Ruby: None (parting). Hornblende: Two distinct cleavages at 56 and 124 degrees.
• Composition: A rock composed of calcium aluminum silicate hydroxide (zoisite), aluminum oxide (ruby/corundum), and complex calcium magnesium iron aluminum silicate hydroxide (hornblende).

Quick Check

• Color: Green (zoisite), Red (ruby), Black (hornblende)
• Luster: Vitreous to pearly (zoisite), Adamantine to vitreous (ruby)
• Streak: White (for zoisite and ruby, though typically not performed on a rock assemblage)

Physical Characteristics

• Crystal Habit: Massive to granular aggregate, with embedded prismatic zoisite and anhedral to subhedral ruby crystals.
• Cleavage Type: Zoisite: Perfect. Ruby: None (parting). Hornblende: Prismatic.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

Formation

Ruby in Zoisite forms through regional metamorphism and metasomatism of chromium-rich mafic or ultramafic rocks (e.g., basalts, gabbros, peridotites) that have been altered to amphibolites or other metamorphic assemblages. The chromium necessary for ruby's red color is typically derived from these host rocks. The zoisite forms from the alteration of plagioclase feldspar, while the ruby (corundum) crystallizes under specific pressure-temperature conditions and in the presence of aluminum and chromium. The black amphibole (often pargasite or hornblende) is also a product of this metamorphic and metasomatic process.

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 Paleozoic, depending on the specific geological setting of metamorphism and metasomatism.

Where to Find

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Scientific Classification

Mineral Class

Silicate (Zoisite, Hornblende), Oxide (Ruby/Corundum)

Group

Epidote Group (Zoisite), Corundum Group (Ruby), Amphibole Group (Hornblende)

Crystal System

Orthorhombic (Zoisite), Hexagonal (Ruby), Monoclinic (Hornblende)

Chemical Formula

Ca2Al3(SiO4)3(OH) (Zoisite), Al2O3 (Ruby), Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2 (Hornblende - generalized)

Composition

A rock assemblage of calcium aluminum silicate hydroxide, aluminum oxide, and complex calcium magnesium iron aluminum silicate hydroxide.

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