Ruby

Description

Ruby is the red variety of the mineral Corundum (aluminum oxide, Al2O3). It is one of the four 'precious' gemstones, alongside diamond, emerald, and sapphire. Its characteristic red color, ranging from pinkish-red to a deep, vivid 'pigeon's blood' red, is caused by the presence of chromium. The intensity of the red color is a primary determinant of its value. Rubies often exhibit fluorescence under ultraviolet light, which can enhance their color. They are known for their exceptional hardness, second only to diamond among natural minerals, making them highly durable.

How to Identify

Color

Varies from pinkish-red to deep, vivid 'pigeon's blood' red. The most valuable rubies have a pure, vibrant red with a slight purplish secondary hue.

Luster

Vitreous (glassy) to adamantine.

Texture

Smooth to slightly uneven on fractured surfaces.

Crystal Form

Typically forms hexagonal prisms, often with flat basal pinacoid terminations. Can also be found as tabular, bipyramidal, or massive forms. Twinning is common.

Cleavage

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

Geological Environment

Metamorphic rocks (marbles, gneisses, schists) and certain igneous rocks (alkali basalts, syenites). Often found in alluvial deposits (placers) due to its hardness and resistance to weathering.

Key Facts

• Hardness: 9 on the Mohs scale
• Specific Gravity: 3.97 - 4.05 g/cm³
• Crystal System: Trigonal
• Color: Red, due to chromium impurities
• Luster: Vitreous to adamantine
• Transparency: Transparent to opaque
• Fracture: Conchoidal to uneven
• Cleavage: None (exhibits parting)
• Composition: Aluminum oxide (Al2O3) with trace chromium (Cr3+)

Quick Check

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

Physical Characteristics

• Crystal Habit: Hexagonal prisms, tabular, bipyramidal, massive. Often twinned.
• Cleavage Type: None, but exhibits parting on basal and rhombohedral planes.
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to adamantine

Formation

Ruby forms in aluminum-rich, silica-poor environments. Primary formation occurs in metamorphic rocks such as marble, gneiss, and schist, where aluminum-rich sediments or igneous rocks undergo high-temperature and high-pressure metamorphism. It can also form in certain igneous rocks, particularly alkali basalts and syenites, through magmatic crystallization or xenocryst incorporation. The red color is due to trace amounts of chromium (Cr3+) substituting for aluminum in the crystal lattice. The presence of iron can lead to brownish or purplish hues, while vanadium can impart a purplish-red.

Usage

Ruby is one of the most valuable gemstones, highly prized for its beauty and durability. It is extensively used in high-end jewelry (rings, necklaces, earrings, bracelets). Due to its hardness (9 on Mohs scale), synthetic ruby is used in industrial applications such as watch bearings, laser components (first successful laser used synthetic ruby), precision instruments, and as an abrasive.

Age Distribution

Rubies form in various geological settings, often associated with metamorphic and igneous rocks, ranging from Precambrian to Cenozoic eras, depending on the specific deposit.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides

Group

Corundum Group

Crystal System

Trigonal

Chemical Formula

Al2O3

Composition

Aluminum oxide with trace chromium (Cr3+) responsible for red color.

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