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Sapphire is a precious gemstone and a variety of the mineral corundum (aluminum oxide, Al₂O₃). While typically known for its striking blue color, sapphire can occur in almost any color except red (red corundum is known as ruby). The color variations are due to trace impurities within the crystal lattice. Its exceptional hardness (9 on the Mohs scale) makes it highly durable and suitable for both jewelry and industrial applications. It crystallizes in the trigonal system, often forming barrel-shaped, bipyramidal, or tabular hexagonal prisms.
How to Identify
- Color
- Typically blue, but can be yellow, pink, orange, green, purple, black, or colorless (white sapphire). The color is often unevenly distributed, appearing as bands or zones.
- Luster
- Vitreous (glassy) to adamantine.
- Texture
- Smooth to slightly rough on crystal faces; often exhibits growth striations.
- Crystal Form
- Hexagonal system, commonly forming barrel-shaped, bipyramidal, tabular, or prismatic crystals. Often found as rounded pebbles in alluvial deposits.
- Cleavage
- No true cleavage, but exhibits parting along basal planes ({0001}) and rhombohedral planes ({1011}) due to lamellar twinning. This parting can resemble cleavage.
- Geological Environment
- Primary deposits are found in metamorphic rocks (e.g., gneisses, schists, marbles), silica-undersaturated igneous rocks (e.g., alkali basalts, nepheline syenites, lamprophyres), and desilicated pegmatites. Secondary (placer) deposits are found in alluvial gravels and riverbeds, where sapphires have been eroded from their primary sources and concentrated due to their high density and hardness.
Key Facts
- Hardness: 9 (Mohs scale)
- Specific Gravity: 3.95 - 4.10 g/cm³
- Crystal System: Trigonal
- Color: Blue, yellow, pink, green, orange, purple, black, colorless (all colors except red)
- Luster: Vitreous to adamantine
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None (exhibits parting)
- Composition: Aluminum oxide (Al₂O₃)
Quick Check
- Color: Blue (most common), but also yellow, pink, green, orange, purple, black, colorless.
- Luster: Vitreous to adamantine.
- Streak: White.
Physical Characteristics
- Crystal Habit: Typically hexagonal prisms, often barrel-shaped, bipyramidal, or tabular. Can be massive or granular.
- Cleavage Type: None, but exhibits parting on {0001} (basal) and {1011} (rhombohedral) planes due to polysynthetic twinning.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous to adamantine
Formation
Sapphire, a variety of corundum, forms under high-temperature and high-pressure conditions. It can crystallize in various geological environments: 1. Metamorphic rocks: During regional or contact metamorphism of aluminum-rich sediments (e.g., bauxites, shales) or igneous rocks, especially in association with desilicated pegmatites or xenoliths in alkali basalts. 2. Igneous rocks: Primarily in silica-undersaturated igneous rocks such as nepheline syenites, alkali basalts, lamprophyres, and anorthosites. It can also form in pegmatites. 3. Hydrothermal alteration: Less commonly, it can form through hydrothermal processes. The blue color in sapphire is typically due to trace amounts of iron (Fe) and titanium (Ti) substituting for aluminum in the crystal lattice, leading to intervalence charge transfer.
Usage
Gemstone (jewelry), abrasive (due to its extreme hardness), watch bearings, scientific instruments (windows, lenses), laser components, and as a substrate for semiconductor devices (e.g., LEDs).
Age Distribution
Precambrian to Cenozoic, depending on geological setting. Primary deposits are often associated with ancient metamorphic and igneous rocks, while secondary alluvial deposits can be much younger.
Where to Find
Sri Lanka (Ceylon)
Famous for a wide range of colors, including the highly prized 'cornflower blue' sapphires, as well as star sapphires. Found in alluvial gravels.
Kashmir, India
Historically renowned for its velvety 'cornflower blue' sapphires, though production is now limited. Found in metamorphic rocks.
Myanmar (Burma)
Produces fine blue sapphires, often associated with ruby deposits. Found in metamorphic and alluvial deposits.
Thailand
Known for dark blue and 'parti-colored' sapphires, often heat-treated. Found in basaltic terrains and alluvial deposits.
Australia
A major producer of dark blue and green sapphires, particularly from Queensland and New South Wales. Found in basaltic fields and alluvial deposits.
Madagascar
Significant source of various colors, including fine blue sapphires. Found in metamorphic and alluvial deposits.
Montana, USA
Produces a range of colors, including blue, green, and yellow, often with a metallic sheen. Found in lamprophyre dikes and alluvial deposits (e.g., Yogo Gulch).
East Africa (Tanzania, Kenya)
Various deposits yielding blue, pink, and 'fancy' colored sapphires. Found in metamorphic and alluvial settings.
Finding Tips
Look in Alluvial Deposits
Many sapphires are found in secondary placer deposits due to their high specific gravity and hardness. Search in riverbeds, ancient stream channels, and gravel pits, especially in regions known for sapphire production. Look for 'heavy minerals' concentrates.
Identify Associated Minerals
In primary deposits, sapphire can be associated with minerals like spinel, garnet, zircon, rutile, feldspar, mica, and tourmaline. In basaltic environments, look for xenocrysts in volcanic bombs or flows.
Check for Crystal Habit
Sapphire often forms distinctive hexagonal barrel-shaped or bipyramidal crystals. Even in alluvial deposits, these shapes may be somewhat preserved, though often rounded.
Test Hardness
Sapphire is extremely hard (Mohs 9). It will scratch almost all other common minerals, including quartz. This is a key diagnostic test, but should be done carefully on a non-gem quality specimen to avoid damage.
Observe Color and Luster
While blue is classic, be aware of other colors. The vitreous to adamantine luster is also characteristic. Look for color zoning or banding, which is common in natural sapphires.
Similar Rocks
Ruby
Corundum (Al₂O₃)
Also known as: Red Corundum
Spinel
MgAl₂O₄
Also known as: Magnesium Aluminum Oxide
Kyanite
Al₂SiO₅
Also known as: Disthene
Benitoite
BaTiSi₃O₉
Also known as: Barium Titanium Silicate
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al₂O₃
- Composition
- Aluminum oxide. Trace elements (e.g., Fe, Ti, Cr, V) cause color variations. Blue is typically due to Fe and Ti.
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