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Jet is a dense, black, opaque, and fine-grained variety of lignite coal. It is distinguished from common lignite by its exceptional hardness, ability to take a brilliant polish, and conchoidal fracture. It is lightweight and warm to the touch, unlike most minerals. It is essentially fossilized wood that has been highly compressed and altered, retaining some of the original woody structure but transformed into a homogeneous, lustrous material.
How to Identify
- Color
- Uniformly deep black, sometimes with a very dark brown tint.
- Luster
- Vitreous (glassy) to waxy when polished; dull to greasy when unpolished.
- Texture
- Smooth and fine-grained. When unpolished, it can feel slightly earthy or woody.
- Crystal Form
- Amorphous; it does not form crystals. It is an organic material, not a mineral.
- Cleavage
- None. It exhibits a conchoidal fracture.
- Geological Environment
- Typically found in marine or estuarine sedimentary rock sequences, often associated with shales, sandstones, and other coal seams. It forms from buried wood in anoxic conditions.
Key Facts
- Hardness: 2.5-4 on the Mohs scale
- Specific Gravity: 1.30-1.35 (unusually low for a gem material, indicating its organic origin)
- Crystal System: Amorphous (non-crystalline)
- Color: Deep black, sometimes with a very dark brown hue
- Luster: Vitreous to waxy (polished), dull to greasy (unpolished)
- Transparency: Opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Primarily carbon (C), with varying amounts of hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). It is a complex organic polymer derived from fossilized wood.
Quick Check
- Color: Deep black
- Luster: Vitreous (polished), dull to greasy (unpolished)
- Streak: Brownish-black
Physical Characteristics
- Crystal Habit: Massive, amorphous
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous (polished), dull to greasy (unpolished)
Formation
Jet forms from the fossilized wood of specific trees, typically Araucariaceae (monkey puzzle family) or similar conifers, that have undergone coalification under specific conditions. The wood is buried in marine or estuarine sediments, where it is compressed and altered by heat and pressure, but without the intense metamorphism that would turn it into higher-grade coal. The presence of saline water during diagenesis is thought to contribute to its characteristic hardness and ability to take a high polish.
Usage
Historically and currently used for ornamental purposes, particularly for carving into jewelry (mourning jewelry was a significant use in the Victorian era), beads, and small decorative objects. It is also used in some traditional crafts.
Age Distribution
Primarily Jurassic (e.g., Whitby Jet) to Tertiary periods, corresponding to major coal-forming epochs.
Where to Find
Whitby, North Yorkshire, England
World-renowned source of high-quality Jet, particularly from the Upper Lias (Jurassic) shales. This is the classic 'Whitby Jet'.
Asturias, Spain
Another historically significant source of high-quality Jet, often found in Carboniferous deposits.
France
Deposits found in various regions, including the Aude department.
Germany
Occurrences in several coal-bearing formations.
Turkey
Known for 'Oltu Stone', a type of jet.
United States
Found in various states, including Utah, Colorado, and New Mexico, often associated with Cretaceous coal beds.
Russia
Deposits in several coal-rich areas.
Finding Tips
Look in Sedimentary Environments
Jet is found in sedimentary rocks, particularly shales and sandstones that were deposited in ancient marine or estuarine environments where wood could be buried and preserved.
Associated with Coal Seams
While distinct from common coal, jet often occurs in proximity to or within coal-bearing strata, especially lignite or sub-bituminous coal deposits.
Check for Lightweight Material
Jet is remarkably lightweight for its size. If you find a black, dense-looking material that feels unusually light, it could be jet.
Observe Fracture and Luster
Look for a conchoidal (shell-like) fracture and, if broken, a dull to greasy luster on the fresh surface. Polished pieces will have a high vitreous luster.
Perform a Scratch Test (Carefully)
Jet is relatively soft (2.5-4 on Mohs scale). It can be scratched by a steel knife. However, this test can damage the specimen, so use with caution on a less visible area.
Hot Needle Test (Destructive)
A definitive but destructive test: a hot needle applied to an inconspicuous part of jet will produce a smell of burning coal/wood. This will not happen with glass or obsidian. Use with extreme caution and only if necessary for identification.
Similar Rocks
Obsidian
Obsidian
Also known as: Volcanic Glass
Black Tourmaline
Schorl (a variety of Tourmaline)
Also known as: Schorl
Onyx
Onyx (a variety of Chalcedony)
Also known as: Banded Chalcedony
Glass
Amorphous solid
Also known as: Manufactured Glass
Common Lignite
Lignite
Also known as: Brown Coal
Scientific Classification
- Mineral Class
- Organic material (not a true mineral)
- Group
- Coal (specifically, a variety of lignite)
- Crystal System
- Amorphous
- Chemical Formula
- Variable, approximately C70H40O2S (representative, not fixed)
- Composition
- Fossilized wood, primarily carbonaceous matter
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