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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 substituting for aluminum in the crystal structure. It is one of the most valuable and sought-after gemstones, renowned for its durability and intense color. The term 'ruby' is reserved exclusively for red corundum; all other color varieties of corundum are classified as sapphire.
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) on crystal faces.
- Texture
- Typically smooth on crystal faces, but can be rough or etched in natural occurrences.
- Crystal Form
- Hexagonal system, commonly forming tabular, prismatic, or bipyramidal crystals. Often found as barrel-shaped or tapering hexagonal prisms.
- Cleavage
- None (parting may be observed along twinning planes or basal planes, but this is not true cleavage).
- Geological Environment
- Metamorphic rocks (marbles, gneisses, schists), silica-undersaturated igneous rocks (basalts, syenites), and alluvial deposits derived from these primary sources.
Key Facts
- Hardness: 9 (Mohs scale)
- Specific Gravity: 3.97 - 4.05
- Crystal System: Hexagonal (Trigonal)
- Color: Red, pinkish-red, purplish-red, brownish-red
- Luster: Vitreous to adamantine
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None (parting sometimes observed)
- 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: Tabular, prismatic, bipyramidal, barrel-shaped, massive, granular
- Cleavage Type: None (parting on {0001} and {1011} due to twinning)
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to adamantine
Formation
Ruby forms under high temperature and pressure conditions, typically in metamorphic rocks (e.g., marble, gneiss, schist) and igneous rocks (e.g., basalt, syenite). The presence of chromium (Cr3+) substituting for aluminum (Al3+) in the crystal lattice is essential for its red color. Formation environments include regional metamorphism of aluminous sediments, contact metamorphism of limestones, and magmatic crystallization in silica-undersaturated igneous rocks.
Usage
Primarily used as a highly prized gemstone in jewelry due to its exceptional hardness, brilliance, and vibrant red color. Industrial applications include use in lasers (synthetic ruby), watch bearings, and precision instruments due to its hardness and wear resistance.
Age Distribution
Rubies form in various geological settings and ages, from Precambrian to Cenozoic, depending on the specific tectonic and metamorphic events.
Where to Find
Mogok Valley, Myanmar (Burma)
Historically the most important source, famous for producing the finest 'pigeon's blood' rubies from marble deposits.
Mong Hsu, Myanmar
A significant source discovered in the 1990s, producing rubies that often require heat treatment to improve color.
Mozambique
Currently a major global source, particularly the Montepuez deposit, producing large quantities of high-quality rubies.
Thailand (Chanthaburi, Trat)
Historically important, known for darker, brownish-red rubies, often found in basaltic terrains.
Sri Lanka (Ratnapura)
Known for producing lighter-colored, often pinkish-red rubies, found in alluvial deposits.
Tanzania (Winza, Longido)
Produces rubies from both metamorphic and igneous sources, including some with excellent color.
Vietnam (Luc Yen, Quy Chau)
Produces fine rubies from marble-hosted deposits, similar to those in Myanmar.
Finding Tips
Look for Associated Minerals
In primary deposits, rubies are often found with minerals like spinel, phlogopite mica, calcite (in marble), and various feldspars. In alluvial deposits, look for heavy mineral concentrates.
Check for Hardness
Ruby is extremely hard (9 on Mohs scale). It will scratch almost all other common minerals except diamond. This can be a key differentiator from similar-looking red minerals.
Observe Crystal Habit
Look for characteristic hexagonal crystal forms, often tabular or barrel-shaped. While many rubies are found as fragments, well-formed crystals are diagnostic.
Fluorescence Test
Many natural rubies, especially those from marble-hosted deposits, exhibit strong red fluorescence under long-wave UV light due to the chromium content. This can help distinguish them from other red gemstones.
Specific Gravity Test
Ruby has a relatively high specific gravity (3.97-4.05). This can be used in conjunction with other tests to differentiate it from lighter minerals.
Similar Rocks
Garnet
Complex Silicate (e.g., Fe3Al2(SiO4)3 for Almandine)
Also known as: Almandine, Pyrope, Spessartine, Grossular, Andradite, Uvarovite
Spinel
MgAl2O4
Also known as: Magnesium Aluminum Oxide
Red Tourmaline
Complex Borosilicate (e.g., Na(Li,Al)3Al6(BO3)3Si6O18(OH)4)
Also known as: Rubellite
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Hexagonal (Trigonal)
- Chemical Formula
- Al2O3
- Composition
- Aluminum Oxide with trace Chromium
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