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Ruby is the red variety of the mineral corundum (aluminum oxide, Al₂O₃). Its distinctive red color is caused by trace amounts of chromium (Cr³⁺) substituting for aluminum in the crystal structure. The intensity of the red color, ranging from pinkish-red to a deep, vivid 'pigeon's blood' red, is directly related to the concentration of chromium. It is one of the hardest natural minerals, second only to diamond, making it exceptionally durable.
How to Identify
- Color
- Ranges from pinkish-red to a deep, vivid 'pigeon's blood' red. The color is often unevenly distributed, appearing as streaks or spots.
- Luster
- Vitreous (glassy) to adamantine (diamond-like) when polished.
- Texture
- Typically smooth on crystal faces, but can be rough on fractured surfaces.
- Crystal Form
- Usually forms as hexagonal prisms with flat basal pinacoids, or as bipyramidal crystals. Can also be found as granular or massive aggregates.
- Cleavage
- No true cleavage, but exhibits parting along twinning planes or basal planes, which can be mistaken for cleavage.
- Geological Environment
- Found in metamorphic rocks such as marble, gneiss, and schist, and in some igneous rocks like basalt. Often occurs in alluvial deposits (placers) due to its hardness and resistance to weathering.
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 (Cr)
Quick Check
- Color: Red (pinkish-red to deep red)
- Luster: Vitreous to adamantine
- Streak: White
Physical Characteristics
- Crystal Habit: Hexagonal prisms, tabular, bipyramidal, granular, massive
- Cleavage Type: None, but exhibits basal and rhombohedral parting
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to adamantine
Formation
Ruby forms in metamorphic rocks (e.g., marble, gneiss, schist) and igneous rocks (e.g., basalt, syenite) under specific conditions of high temperature and pressure, and the presence of aluminum and chromium. The chromium replaces some of the aluminum ions in the corundum crystal lattice, causing the characteristic red color. It often forms in environments where aluminum-rich rocks interact with chromium-bearing fluids or rocks.
Usage
Primarily used as a gemstone in jewelry due to its hardness, brilliance, and vibrant red color. High-quality rubies are among the most valuable gemstones. Industrial applications of synthetic corundum (including synthetic ruby) include abrasives, laser components (ruby lasers), and precision bearings.
Age Distribution
Rubies are found in geological formations ranging from Precambrian to Cenozoic, with many significant deposits being hundreds of millions to over a billion years old.
Where to Find
Myanmar (Burma)
Historically the most important source, particularly the Mogok Valley, famous for producing the highly prized 'pigeon's blood' rubies found in marble deposits.
Thailand
Known for darker, brownish-red rubies, often found in basaltic host rocks. Chanthaburi and Trat provinces are key areas.
Sri Lanka
Produces a range of ruby colors, often lighter red to pinkish-red, found in alluvial deposits derived from metamorphic rocks.
Mozambique
A significant modern source, particularly the Montepuez deposit, producing high-quality rubies from amphibolite-hosted deposits.
Afghanistan
Historically known for rubies from the Jegdalek deposit, often found in marble.
Vietnam
Luc Yen and Quy Chau districts are known for rubies found in marble and alluvial deposits.
Tanzania
Various localities, including Winza and Longido, produce rubies, often with a purplish tint, from metamorphic rocks.
Finding Tips
Look for associated minerals
In primary deposits, rubies are often found with minerals like spinel, phlogopite, calcite, graphite, and various feldspars, depending on the host rock.
Check alluvial deposits
Due to its high specific gravity and hardness, ruby often concentrates in riverbeds and ancient placer deposits. Look for gravels containing heavy minerals.
Examine host rocks
In metamorphic terrains, search for marble, gneiss, or schist outcrops. In igneous settings, look for basalt or syenite. Ruby crystals may be embedded within these rocks.
Use a loupe or magnifying glass
Rubies often have characteristic inclusions (e.g., rutile needles 'silk', growth zoning, fingerprints) that can help distinguish them from imitations or other red minerals.
Test hardness
Ruby is very hard (9 on Mohs scale). It will scratch quartz (7) and feldspar (6-6.5). This is a key differentiating factor from many other red minerals.
Similar Rocks
Garnet
Complex Silicate (e.g., Fe₃Al₂(SiO₄)₃ for Almandine)
Also known as: Almandine, Pyrope, Spessartine, Grossular, Andradite, Uvarovite
Red Spinel
Magnesium Aluminum Oxide (MgAl₂O₄)
Also known as: Balas Ruby
Red Tourmaline
Complex Borosilicate (e.g., Na(Li,Al)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ for Rubellite)
Also known as: Rubellite
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al₂O₃:Cr
- Composition
- Aluminum oxide with chromium as the chromophore
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