Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Ruby is the red variety of the mineral Corundum (aluminum oxide, Al2O3). Its distinctive red color is caused by trace amounts of chromium. It is one of the hardest natural minerals, second only to diamond, making it highly durable. Rubies can range in color from pinkish-red to a deep, vivid 'pigeon's blood' red, which is the most prized hue. They often exhibit fluorescence under ultraviolet light, which can enhance their color.
How to Identify
- Color
- Ranges from pinkish-red to deep, vivid 'pigeon's blood' red. The color is due to chromium impurities.
- Luster
- Vitreous (glassy) to adamantine.
- Texture
- Typically smooth on crystal faces, but can be rough in unworked specimens. Fracture is conchoidal to uneven.
- Crystal Form
- Hexagonal system, typically forming tabular, prismatic, or bipyramidal crystals. Often found as barrel-shaped or tapering hexagonal prisms.
- Cleavage
- No true cleavage, but exhibits parting along basal planes and rhombohedral planes due to lamellar twinning.
- Geological Environment
- Found in metamorphic rocks such as marble, gneiss, and schist, and in some igneous rocks like basalt. Also commonly found in alluvial placer deposits due to its hardness and resistance to weathering.
Key Facts
- Hardness: 9 (Mohs scale)
- Specific Gravity: 3.97 - 4.05
- Crystal System: Trigonal (Hexagonal system, rhombohedral division)
- 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 tabular, prismatic, or bipyramidal hexagonal crystals; also granular, massive, or lamellar.
- Cleavage Type: None, but exhibits parting on {0001} (basal) and {1011} (rhombohedral) planes due to polysynthetic twinning.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to adamantine.
Formation
Ruby forms under high temperature and pressure conditions 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 responsible for its characteristic red color. It often forms in environments where aluminum is abundant and silica is deficient, preventing the formation of feldspars or quartz. Metasomatic processes, where fluids introduce chromium into corundum-forming environments, are also crucial for ruby formation.
Usage
Primarily used as a precious gemstone in jewelry due to its hardness, brilliance, and intense red color. Historically, it has also been used in watch bearings, scientific instruments, and as a laser medium (synthetic ruby).
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)
Renowned for producing some of the finest 'pigeon's blood' rubies, primarily from marble-hosted deposits and associated placer deposits.
Mong Hsu, Myanmar
A significant source of rubies, often requiring heat treatment to improve color and clarity.
Thailand (Chanthaburi, Trat)
Historically a major source, particularly of darker, brownish-red rubies, often found in basalt-related deposits.
Sri Lanka (Ratnapura)
Known for producing rubies of various shades, often found in alluvial deposits alongside other gemstones.
Mozambique
A relatively new but highly significant source, producing a wide range of ruby qualities, often from primary metamorphic deposits and secondary placers.
Tanzania (Winza, Longido)
Produces rubies from both metamorphic and metasomatic deposits, sometimes with distinctive color zoning.
Vietnam (Luc Yen, Quy Chau)
Produces fine rubies from marble-hosted deposits, similar to those in Myanmar.
Finding Tips
Look in Alluvial Deposits
Due to its high specific gravity and hardness, ruby often concentrates in riverbeds and ancient stream channels. Look for gravels containing other heavy minerals.
Identify Associated Minerals
In primary deposits, ruby can be found with minerals like spinel, phlogopite mica, calcite, graphite, and various amphiboles or pyroxenes, depending on the host rock.
Check for Fluorescence
Many natural rubies fluoresce red under long-wave UV light, which can help distinguish them from other red minerals. However, not all rubies fluoresce strongly, and some synthetics also fluoresce.
Test Hardness
Ruby is very hard (9 on Mohs scale). It will scratch quartz (7) and topaz (8). This is a key diagnostic property, but should be done carefully on a non-gem quality part of the specimen.
Observe Crystal Habit
Look for characteristic hexagonal crystal forms, often tabular or prismatic, which can be distinctive even in rough specimens.
Similar Rocks
Red Spinel
Spinel (MgAl2O4)
Also known as: Balas Ruby
Garnet
Garnet Group (e.g., Almandine Fe3Al2(SiO4)3, Pyrope Mg3Al2(SiO4)3)
Also known as: Almandine, Pyrope
Red Tourmaline
Elbaite (Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)4)
Also known as: Rubellite
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al2O3
- Composition
- Aluminum oxide with trace chromium (Cr3+)
Explore Ruby
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.