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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 the presence of trace amounts of chromium. It is one of the 'Big Four' gemstones, alongside diamonds, emeralds, and sapphires (which are also corundum but of different colors). Ruby is highly prized for its hardness, durability, luster, and rarity, particularly in larger, high-quality specimens. It often exhibits strong fluorescence under ultraviolet light, which can enhance its color and is a characteristic used in identification.
How to Identify
- Color
- Ranges from pinkish-red to deep, vivid 'pigeon's blood' red. The color is typically unevenly distributed, often concentrated in patches or streaks.
- Luster
- Vitreous (glassy) to adamantine.
- Texture
- Typically smooth to the touch, with a dense, compact feel.
- Crystal Form
- Hexagonal system, commonly found as tabular, prismatic, or bipyramidal crystals. Often twinned.
- Cleavage
- No true cleavage, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning.
- Geological Environment
- Found in metamorphic rocks (marbles, gneisses, schists), igneous rocks (basalts, syenites), and secondary alluvial deposits derived from these primary sources.
Key Facts
- Hardness: 9 on the Mohs scale
- Specific Gravity: 3.97 - 4.05
- Crystal System: Hexagonal (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 (Cr)
Quick Check
- Color: Red (pinkish-red to deep red)
- Luster: Vitreous to adamantine
- Streak: White
Physical Characteristics
- Crystal Habit: Typically prismatic, tabular, or bipyramidal crystals; also granular, massive, or in lamellar aggregates. Often twinned.
- Cleavage Type: No true cleavage; exhibits parting along basal planes {0001} and rhombohedral planes {1011} due to lamellar twinning.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous to adamantine
Formation
Ruby forms in a variety of geological environments, primarily in metamorphic rocks (e.g., marble, gneiss, schist) and igneous rocks (e.g., basalt, syenite). The presence of chromium (Cr³⁺) as a trace element substituting for aluminum (Al³⁺) in the corundum crystal lattice is essential for the red color. Formation typically occurs under high-temperature and high-pressure conditions. In metamorphic settings, it often forms through the metamorphism of aluminum-rich sediments or bauxites, or through metasomatic processes where chromium-bearing fluids interact with aluminum-rich rocks. In igneous settings, it can crystallize directly from silica-undersaturated magmas or form in xenoliths carried to the surface by volcanic activity. Secondary alluvial deposits, formed by the weathering and erosion of primary sources, are also significant.
Usage
Ruby is one of the most valuable and sought-after gemstones, primarily used in high-end jewelry. Its exceptional hardness (9 on Mohs scale) also makes industrial-grade corundum (including some non-gem quality ruby) useful in applications requiring abrasion resistance, such as in watch bearings, precision instruments, and as an abrasive material. Synthetic rubies are used in lasers (e.g., ruby lasers) and for industrial purposes.
Age Distribution
Rubies are found in geological formations ranging from Precambrian to Cenozoic, with many significant deposits associated with metamorphic events from the Paleozoic and Mesozoic eras.
Where to Find
Mogok Valley, Myanmar (Burma)
Historically the most famous source, known for producing the finest 'pigeon's blood' rubies from marble-hosted deposits. These rubies are highly fluorescent.
Mong Hsu, Myanmar
A significant source discovered in the 1990s, producing rubies that often require heat treatment to improve color and clarity.
Mozambique
Currently a major global source, particularly from the Montepuez deposit, producing a wide range of ruby qualities, often with good clarity and color.
Thailand (Chanthaburi, Trat)
Historically important, known for darker, brownish-red rubies, often found in basaltic host rocks. Many Thai rubies were heat-treated.
Sri Lanka (Ratnapura)
Known for producing lighter, pinkish-red rubies, often found in alluvial deposits alongside sapphires and other gemstones.
Vietnam (Luc Yen, Quy Chau)
Produces high-quality rubies, often from marble-hosted deposits, similar to those in Myanmar.
Tanzania (Winza, Longido)
Winza is known for its vibrant, often purplish-red rubies. Longido produces rubies in zoisite.
Madagascar
Various deposits, producing rubies of different qualities and colors, often requiring heat treatment.
Finding Tips
Primary Deposits
Look for ruby in metamorphic terrains, particularly in marble, gneiss, or schist, especially where there is evidence of chromium-rich minerals or metasomatic alteration. Igneous occurrences are less common but can be found in silica-undersaturated rocks like basalt or syenite.
Secondary (Alluvial) Deposits
Many rubies are found in placer deposits (riverbeds, ancient terraces) due to their high specific gravity and durability. Search in gravels and sands downstream from known primary sources. Panning and sieving techniques are often employed.
Associated Minerals
Ruby can be found alongside minerals such as spinel, garnet, phlogopite mica, graphite, calcite, and various feldspars, depending on the geological environment.
Color and Luster
Identify potential ruby by its characteristic red color and vitreous to adamantine luster. Even in rough form, the color should be distinct. Use a strong light source to observe its transparency and potential fluorescence.
Hardness Test (Caution)
While not recommended for valuable specimens, a scratch test against known minerals can help. Ruby (Mohs 9) will scratch quartz (Mohs 7) and topaz (Mohs 8). Be careful not to damage the specimen.
Similar Rocks
Garnet
Complex silicate minerals (e.g., Fe₃Al₂(SiO₄)₃ for Almandine)
Also known as: Almandine, Pyrope, Spessartine, Grossular, Andradite, Uvarovite
Spinel
MgAl₂O₄
Also known as: Magnesium aluminum oxide
Topaz
Al₂SiO₄(F,OH)₂
Also known as: Aluminum fluorosilicate
Tourmaline (Rubellite)
Complex borosilicate (e.g., Na(Li,Al)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ for Elbaite)
Also known as: Elbaite (red variety)
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Hexagonal (Trigonal)
- Chemical Formula
- Al₂O₃
- Composition
- Aluminum oxide with trace amounts of chromium (Cr³⁺) responsible for the red color. Other trace elements like iron (Fe) and titanium (Ti) can influence color and fluorescence.
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