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Fossilized coral is a type of fossil where the original organic material of ancient corals has been replaced by minerals, most commonly silica. The resulting fossil retains the characteristic patterns and structures of the original coral colony or individual polyp, such as septa, tabulae, and corallites. Rugosa corals (horn corals) are typically solitary or colonial with prominent septa, while Tabulata corals are exclusively colonial, forming honeycomb-like or chain-like structures with well-developed tabulae and lacking septa. The fossilization process often results in a durable, colorful material that can be polished to a high luster.
How to Identify
- Color
- Highly variable, depending on the trace minerals present during silicification. Common colors include white, gray, brown, black, pink, red, yellow, and blue. Often exhibits banding or concentric patterns.
- Luster
- Vitreous to waxy, especially when polished.
- Texture
- Microcrystalline to cryptocrystalline, often with a smooth feel when polished. Unpolished surfaces can be rough or granular. The defining characteristic is the preserved internal structure of the coral, visible as intricate patterns.
- Crystal Form
- Cryptocrystalline (chalcedony) or microcrystalline (quartz). Individual crystals are typically too small to be seen without magnification.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Marine sedimentary environments, particularly ancient shallow seas, reefs, and lagoons where corals thrived. Found within limestone, shale, or sandstone formations that have undergone silicification.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification)
- Specific Gravity: 2.58-2.64 (for chalcedony/quartz)
- Crystal System: Trigonal (for quartz/chalcedony)
- Color: Highly variable, often with preserved coral patterns
- Luster: Vitreous to waxy
- Transparency: Translucent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2) after silicification, with trace impurities causing color variations. Original coral was calcium carbonate (CaCO3).
Quick Check
- Color: Variable (white, gray, brown, black, pink, red, yellow, blue)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, forming botryoidal, mammillary, or stalactitic masses, or filling voids and replacing organic structures.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Fossilized coral forms when ancient coral skeletons, originally composed of calcium carbonate (aragonite or calcite), are replaced by silica (chalcedony or quartz) through a process called permineralization or replacement. This typically occurs in marine sedimentary environments where corals lived, died, and were subsequently buried by sediments. Groundwater rich in dissolved silica then infiltrates the porous coral structure, slowly replacing the original calcium carbonate with silica while preserving the intricate skeletal details.
Usage
Primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, and decorative objects. It is also valued by fossil collectors and paleontologists for scientific study, providing insights into ancient marine ecosystems and paleoclimates.
Age Distribution
Paleozoic Era (approximately 541 to 252 million years ago) for Rugosa and Tabulata corals. Younger fossil corals (e.g., Scleractinia) also exist but are not the primary focus here.
Where to Find
Florida, USA
Known for its agatized coral, particularly the 'Florida Agatized Coral' which is the state stone. Found in Miocene-Pliocene age deposits.
Indonesia
Produces a wide variety of fossilized corals, often with vibrant colors and intricate patterns, from various geological periods.
Morocco
Rich in Paleozoic fossil corals, including Rugosa and Tabulata forms, often found in limestone matrices.
United Kingdom
Devonian and Carboniferous limestones contain numerous fossil corals, particularly in areas like Devon and the Peak District.
Australia
Various locations yield fossil corals, including silicified specimens, from different geological eras.
Finding Tips
Look in Sedimentary Rocks
Fossilized corals are typically found in sedimentary rock formations, especially limestones, shales, and sandstones that were deposited in ancient marine environments.
Identify Coral Structures
Search for characteristic coral patterns: radial septa (Rugosa), honeycomb or chain-like structures with tabulae (Tabulata), or other colonial growth forms. These patterns are often visible on weathered surfaces or in broken pieces.
Check for Silicification
Silicified corals will often be harder and more resistant to weathering than the surrounding matrix. They may also exhibit a waxy or vitreous luster when broken or polished.
Visit Known Fossil Localities
Research local geological maps and fossil hunting guides for areas known to contain marine fossils, particularly those from the Paleozoic Era for Rugosa and Tabulata corals.
Safety Precautions
Always obtain permission before collecting on private land. Wear appropriate safety gear, including gloves and eye protection, especially when breaking rocks. Be aware of your surroundings and potential hazards in the field.
Similar Rocks
Petrified Wood
Silicified Wood
Also known as: Fossilized Wood
Agate
Cryptocrystalline Quartz
Also known as: Chalcedony
Limestone (bioclastic)
Calcium Carbonate Rock
Also known as: Fossiliferous Limestone
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, when silicified)
- Group
- Quartz group (chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (after silicification)
- Composition
- Silicon dioxide, often with microscopic inclusions of other minerals (e.g., iron oxides, manganese oxides) that impart color.
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