Faceted Gemstone

Mineral

Unknown Gemstone (possibly Sapphire or Spinel based on color and cut)

Also known as: Unknown Gemstone, Blue Gemstone

Got a photo? Identify rocks instantly with the Rockby app

Description

This entry describes a faceted gemstone of unknown identity, but with characteristics (color, cut) suggesting it could be either sapphire (a variety of corundum) or spinel. Both minerals can exhibit similar blue hues and are commonly faceted for jewelry. A definitive identification requires advanced gemological testing.

How to Identify

Color: Typically blue, ranging from light sky blue to deep royal blue. Other colors are possible for both sapphire and spinel, but the context suggests blue.
Luster: Vitreous (glassy) to adamantine.
Texture: Smooth, polished surface due to faceting.
Crystal Form: Faceted, meaning it has been cut and polished into a specific geometric shape to enhance its brilliance and color. Natural crystal forms for sapphire are typically hexagonal prisms or bipyramids; for spinel, they are commonly octahedra or dodecahedra.
Cleavage: Sapphire has no true cleavage but exhibits parting along basal planes or rhombohedral planes. Spinel has imperfect octahedral cleavage, but it is rarely observed in faceted stones.
Geological Environment: Primary deposits in metamorphic rocks (marbles, gneisses, schists) and igneous rocks (basalts, syenites, peridotites). Secondary alluvial deposits are common for both due to their durability.

Key Facts

Hardness: Sapphire: 9 (Mohs); Spinel: 8 (Mohs)
Specific Gravity: Sapphire: 3.95-4.03; Spinel: 3.58-3.61
Crystal System: Sapphire: Trigonal; Spinel: Isometric (Cubic)
Color: Blue (most common for this description), but can be various colors for both minerals.
Luster: Vitreous to adamantine
Transparency: Transparent to translucent
Fracture: Conchoidal to uneven
Cleavage: Sapphire: No true cleavage, but parting; Spinel: Imperfect octahedral cleavage
Composition: Sapphire: Aluminum Oxide (Al2O3); Spinel: Magnesium Aluminum Oxide (MgAl2O4)

Quick Check

Color: Blue (various shades)
Luster: Vitreous to adamantine
Streak: White (for both sapphire and spinel)

Physical Characteristics

Crystal Habit: Faceted (cut from natural crystals which are typically hexagonal prisms/bipyramids for sapphire, and octahedra/dodecahedra for spinel)
Cleavage Type: Sapphire: Parting on {0001} and {1011}; Spinel: Imperfect octahedral on {111}
Fracture Type: Conchoidal to uneven
Tenacity: Brittle
Luster Type: Vitreous to adamantine

Formation

Sapphire (Corundum) forms in aluminum-rich, silica-poor metamorphic or igneous rocks, such as gneisses, schists, and syenites, or as xenocrysts in basaltic lavas. Spinel forms in metamorphic rocks (marbles, gneisses, schists) and mafic igneous rocks (peridotites, gabbros). Both can be found in secondary alluvial deposits due to their hardness and resistance to weathering.

Usage

Primarily as a gemstone in jewelry due to its beauty, hardness, and durability. Historically, sapphire has been used in industrial applications (e.g., watch bearings, scientific instruments) due to its extreme hardness.

Age Distribution

Precambrian to Cenozoic, depending on host rock formation

Where to Find

Sri Lanka

Renowned for both sapphire and spinel, particularly from alluvial deposits in the Ratnapura district.

Myanmar (Burma)

Famous for high-quality rubies and sapphires, and also for fine red and blue spinels, especially from the Mogok Stone Tract.

Madagascar

A significant source of sapphires, particularly from the Ilakaka and Andranondambo regions. Spinel is also found.

Thailand/Cambodia

Historically important sources of blue sapphires, often found in basaltic terrains.

Australia

Known for dark blue sapphires, particularly from Queensland and New South Wales.

United States (Montana)

Produces various colors of sapphire, including blue, from Yogo Gulch and other localities.

Vietnam

Emerging source of both sapphire and spinel.

Finding Tips

Professional Gemological Testing

The most reliable method for distinguishing between sapphire and spinel (or other blue gemstones) is through professional gemological testing, which includes refractive index (RI), specific gravity (SG), spectroscopy, and polariscopy. Sapphire is doubly refractive, while spinel is singly refractive.

Hardness Test (Destructive)

While not recommended for a faceted gemstone, sapphire (Mohs 9) is harder than spinel (Mohs 8). This test would damage the stone.

Inclusions

Examining internal inclusions under magnification can provide clues. Sapphire often exhibits rutile needles ('silk'), growth zoning, and fingerprints. Spinel may show octahedral crystals or stress fractures.

Pleochroism

Sapphire is pleochroic (shows different colors when viewed from different angles), while spinel is not. This can be observed with a dichroscope.

Similar Rocks

Sapphire

Al2O3 (Corundum)

Also known as: Blue Corundum

Spinel

MgAl2O4

Also known as: Magnesium Aluminum Oxide

Blue Topaz

Al2SiO4(F,OH)2

Also known as: Aluminum Silicate Fluoride Hydroxide

Tanzanite

Ca2Al3(SiO4)3(OH)

Also known as: Blue Zoisite

Scientific Classification

Mineral Class: Sapphire: Oxide; Spinel: Oxide (Spinel Group)
Group: Sapphire: Corundum Group; Spinel: Spinel Group
Crystal System: Sapphire: Trigonal; Spinel: Isometric
Chemical Formula: Sapphire: Al2O3; Spinel: MgAl2O4
Composition: Sapphire: Aluminum oxide, often with trace elements (e.g., Fe, Ti for blue); Spinel: Magnesium aluminum oxide, with possible substitutions (e.g., Fe, Zn, Cr)

Explore Faceted Gemstone

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app