Ruby

Description

Ruby is the red variety of the mineral corundum (aluminum oxide, Al₂O₃). Its distinctive red color, ranging from pinkish-red to a deep, vivid 'pigeon's blood' red, is caused by trace amounts of chromium. It is one of the hardest natural minerals, second only to diamond, making it extremely durable. Ruby crystallizes in the trigonal system, typically forming hexagonal prisms, tabular crystals, or bipyramidal forms. It exhibits a vitreous to adamantine luster and is often fluorescent under ultraviolet light. The presence of rutile needles can cause asterism, leading to 'star rubies'.

How to Identify

Color

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

Luster

Vitreous to adamantine.

Texture

Typically smooth on crystal faces, conchoidal fracture surfaces.

Crystal Form

Hexagonal prisms, tabular crystals, bipyramidal forms. Often found as irregular grains in alluvial deposits.

Cleavage

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

Geological Environment

Metamorphic rocks (marble, gneiss, schist), igneous rocks (basalt, syenite, pegmatites), and secondary alluvial deposits.

Key Facts

• Hardness: 9 (Mohs scale)
• Specific Gravity: 3.97 - 4.05 g/cm³
• Crystal System: 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

Quick Check

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

Physical Characteristics

• Crystal Habit: Hexagonal prisms, tabular, bipyramidal; also granular, massive, or in alluvial pebbles.
• Cleavage Type: None, but exhibits parting along {0001} (basal) and {1011} (rhombohedral) planes due to lamellar twinning.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to adamantine.

Formation

Ruby forms in aluminum-rich, silica-poor metamorphic rocks such as marble, gneiss, and schist, or in igneous rocks like basalt and syenite. The presence of chromium (Cr³⁺) as a trace impurity substituting for aluminum (Al³⁺) in the crystal lattice is responsible for its characteristic red color. Formation typically occurs under high temperature and pressure conditions. In metamorphic environments, it often forms through the recrystallization of aluminum-rich sediments or alteration of igneous rocks. In igneous environments, it can crystallize directly from magmas or form in pegmatites. Secondary alluvial deposits, where rubies are weathered out of their primary host rocks and concentrated in riverbeds, are also significant sources.

Usage

Primarily used as a highly prized gemstone in jewelry due to its exceptional hardness, brilliance, and vibrant red color. High-quality rubies are among the most valuable gemstones. Lower-quality or synthetic rubies are used in industrial applications such as watch bearings, laser components (e.g., ruby lasers), and precision instruments due to their hardness and optical properties.

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

Trigonal

Chemical Formula

Al₂O₃

Composition

Aluminum oxide with trace amounts of chromium (Cr³⁺) responsible for the red color. Iron (Fe²⁺, Fe³⁺) and titanium (Ti⁴⁺) can also be present, influencing color and transparency.

Explore Ruby