Ruby

Description

Ruby is a variety of the mineral corundum (aluminum oxide, Al2O3) that exhibits a distinct red color due to the presence of trace amounts of chromium. It is one of the four precious gemstones, along with diamond, emerald, and sapphire (which is also a variety of corundum, but in other colors). The color can range from pinkish-red to a deep, vivid 'pigeon's blood' red, which is the most highly valued. Its exceptional hardness makes it very durable and resistant to scratching, second only to diamond among natural minerals. Rubies often exhibit fluorescence under ultraviolet light, which can enhance their color. Inclusions, such as rutile needles ('silk'), are common and can sometimes create a star effect (asterism) when cut en cabochon.

How to Identify

Color

Varies from pinkish-red to deep, vivid 'pigeon's blood' red. The color is due to chromium impurities.

Luster

Vitreous (glassy) to adamantine.

Texture

Typically smooth on crystal faces, but can be rough in unpolished specimens. Fracture is conchoidal.

Crystal Form

Hexagonal system, commonly found as hexagonal prisms, tabular crystals, or bipyramidal forms. Often twinned.

Cleavage

None, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning.

Geological Environment

Metamorphic rocks (marble, gneiss, schist), igneous rocks (basalt, pegmatite), and alluvial deposits.

Key Facts

• Hardness: 9 (Mohs scale)
• Specific Gravity: 3.97 - 4.05
• Crystal System: Hexagonal (trigonal)
• Color: Red (due to Cr3+ impurities)
• Luster: Vitreous to adamantine
• Transparency: Transparent to opaque
• Fracture: Conchoidal to uneven
• Cleavage: None (exhibits parting)
• Composition: Aluminum oxide (Al2O3) with trace chromium (Cr)

Quick Check

• Color: Red (pinkish-red to deep red)
• Luster: Vitreous to adamantine
• Streak: White

Physical Characteristics

• Crystal Habit: Hexagonal prisms, tabular crystals, bipyramidal, massive, granular
• Cleavage Type: None, but exhibits parting on {0001} and {1011} due to lamellar twinning
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to adamantine

Formation

Ruby forms in a variety of geological environments, primarily in metamorphic rocks such as marble, gneiss, and schist, and in igneous rocks like basalt and pegmatite. The presence of chromium (Cr3+) substituting for aluminum (Al3+) in the corundum crystal lattice is responsible for its characteristic red color. Formation typically occurs under high temperature and pressure conditions. In marble-hosted deposits, rubies form through metasomatic processes where aluminum-rich fluids interact with carbonate rocks. In basalt-hosted deposits, rubies crystallize directly from aluminum-rich, silica-undersaturated magmas. Alluvial deposits, formed by the weathering and erosion of primary sources, are also significant.

Usage

Ruby is one of the most valuable gemstones, highly prized for its hardness, durability, and intense red color. It is extensively used in jewelry, including rings, necklaces, earrings, and bracelets. Due to its exceptional hardness (9 on the Mohs scale), it is also used in industrial applications such as watch bearings, precision instruments, and as an abrasive, though synthetic corundum is more commonly used for these purposes. High-quality rubies are also sought after by collectors and investors.

Age Distribution

Rubies are found in geological formations ranging from Precambrian to Cenozoic, with significant deposits often associated with metamorphic events or magmatic intrusions that occurred millions to hundreds of millions of years ago.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides

Group

Corundum Group

Crystal System

Hexagonal (trigonal)

Chemical Formula

Al2O3:Cr

Composition

Aluminum oxide with chromium impurities

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