Ruby

Description

Ruby is the red variety of the mineral corundum (aluminum oxide, Al2O3). Its vibrant red color, ranging from pinkish-red to a deep, blood-red, is caused by trace amounts of chromium. It is one of the hardest natural minerals, second only to diamond, making it exceptionally durable. Ruby typically forms as prismatic, tabular, or bipyramidal hexagonal crystals. It exhibits a vitreous to adamantine luster and is transparent to opaque. The most prized rubies possess a vivid, saturated red color often referred to as 'pigeon's blood' red. Rubies often display fluorescence under ultraviolet light, which can enhance their color and is a diagnostic feature.

How to Identify

Color

Varies from pinkish-red to deep, vivid blood-red. The most valuable rubies are a pure, saturated red with a slight purplish secondary hue.

Luster

Vitreous to adamantine.

Texture

Smooth to slightly rough on crystal faces; conchoidal fracture surfaces are smooth and glassy.

Crystal Form

Typically forms as hexagonal prisms, often with flat basal pinacoids or bipyramidal terminations. Can also be found as granular or massive aggregates.

Cleavage

No true cleavage, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning. This parting is not as clean or easy as true cleavage.

Geological Environment

Found in metamorphic rocks (marbles, gneisses, schists), igneous rocks (basalts, syenites), and alluvial placer deposits derived from these primary sources.

Key Facts

• Hardness: 9 (Mohs scale)
• Specific Gravity: 3.97 - 4.05
• 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

Quick Check

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

Physical Characteristics

• Crystal Habit: Prismatic, tabular, bipyramidal, granular, massive
• Cleavage Type: None (parting on {0001} and {1011})
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to adamantine

Formation

Ruby, a variety of the mineral corundum, forms under high-temperature and high-pressure conditions. It typically crystallizes in metamorphic rocks such as marble, gneiss, and schist, where aluminum-rich sediments or igneous rocks undergo regional or contact metamorphism. It can also form in certain igneous environments, particularly in silica-poor, aluminum-rich magmas like nepheline syenites or alkali basalts. The characteristic red color is due to trace amounts of chromium (Cr3+) substituting for aluminum (Al3+) in the crystal lattice. The presence of iron can lead to brownish or purplish tints, while vanadium can impart a purplish-red hue. The absence of iron is crucial for the purest red colors.

Usage

Ruby is one of the most valuable gemstones, highly prized for its intense red color, hardness, and brilliance. It is extensively used in high-end jewelry, including rings, necklaces, earrings, and bracelets. Historically, rubies were also used as talismans and symbols of power and passion. Due to its exceptional hardness (9 on the Mohs scale), corundum (including ruby) is used as an abrasive in industrial applications, such as grinding wheels, sandpaper, and polishing compounds. Synthetic rubies are used in lasers (e.g., ruby lasers), watch bearings, and other precision instruments.

Age Distribution

Rubies are found in geological formations ranging from Precambrian to Cenozoic, with significant deposits in metamorphic rocks (e.g., marbles, gneisses) and igneous rocks (e.g., basalts, syenites). Their formation is tied to specific geological events rather than a narrow age range.

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 (Cr) responsible for red color

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