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Fossilized coral, specifically from the extinct orders Rugosa and Tabulata, represents the preserved skeletal remains of ancient marine invertebrates. These corals built colonies or solitary structures of calcium carbonate. Over geological time, these structures were buried and underwent diagenesis, where the original mineralogy was replaced by silica, resulting in a durable, often colorful, fossil. The characteristic patterns seen in fossilized coral are the preserved internal structures of the coral polyps, such as septa (in Rugosa) or tabulae and corallites (in Tabulata).
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 multiple colors in banded or mottled patterns.
- Luster
- Waxy to dull, sometimes vitreous (glassy) if highly polished.
- Texture
- Smooth to slightly granular. The surface often reveals the intricate patterns of the original coral structure.
- Crystal Form
- Cryptocrystalline (microscopic crystals of quartz/chalcedony), preserving the macroscopic form of the original coral colony or solitary polyp.
- Cleavage
- None, as it is composed of cryptocrystalline quartz/chalcedony.
- Geological Environment
- Found in marine sedimentary rock formations that were once ancient coral reefs. These environments are typically shallow, warm, clear waters conducive to coral growth, followed by burial and silicification.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification)
- Specific Gravity: 2.58-2.64 (similar to quartz/chalcedony)
- Crystal System: Trigonal (for the replacing quartz/chalcedony)
- Color: Highly variable, often banded or mottled
- Luster: Waxy to dull
- Transparency: Opaque to translucent
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2), replacing original calcium carbonate (CaCO3)
Quick Check
- Color: Variable (white, gray, brown, black, pink, red, yellow, blue)
- Luster: Waxy to dull, sometimes vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline, preserving original coral morphology
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy to dull
Formation
Fossilized coral forms when the original calcium carbonate (aragonite or calcite) skeletal material of ancient corals is replaced by silica (chalcedony or quartz) through a process called permineralization or replacement. This typically occurs in marine sedimentary environments where silica-rich waters percolate through buried coral reefs. The intricate internal structures of the coral polyps are often preserved in exquisite detail.
Usage
Primarily used as an ornamental stone for jewelry, carvings, and decorative objects. Due to its unique patterns and historical significance, it is also highly valued by fossil collectors and paleontologists for scientific study.
Age Distribution
Rugosa: Middle Ordovician to Late Permian; Tabulata: Early Ordovician to Late Permian
Where to Find
Florida, USA
Known for its agatized coral, particularly from the Tampa Bay area, often found in Miocene-Pliocene age formations.
Indonesia
Produces a wide variety of fossilized corals, often with vibrant colors and intricate patterns, from various Cenozoic marine deposits.
Morocco
Significant source of Devonian-age fossilized Rugosa corals, often found in limestone and shale formations.
Australia
Various locations yield fossilized corals, including Permian and Devonian examples, often found in sedimentary basins.
United Kingdom
Carboniferous and Devonian coral fossils are found in limestone outcrops, particularly in areas like the Mendip Hills and parts of Scotland.
Finding Tips
Geological Maps
Consult geological maps to identify areas with marine sedimentary rocks of Ordovician to Permian age, as these are the most likely locations for Rugosa and Tabulata corals.
Coastal and River Exposures
Look for exposures along coastlines, riverbeds, and road cuts where older sedimentary layers are exposed.
Quarries and Construction Sites
Permission-based access to quarries or construction sites that excavate marine sedimentary rocks can be fruitful.
Examine Rock Surfaces
Look for distinctive patterns on weathered rock surfaces that resemble honeycomb (Tabulata) or horn-like structures with radial septa (Rugosa).
Associated Fossils
Fossilized corals are often found alongside other marine fossils such as brachiopods, crinoids, and trilobites, indicating a marine depositional environment.
Similar Rocks
Modern Coral
Scleractinia (extant order)
Also known as: Living Coral, Coral Skeleton
Chert
Microcrystalline Quartz
Also known as: Flint, Jasper
Agate
Cryptocrystalline Quartz
Also known as: Banded Chalcedony
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz/chalcedony)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (replacing original CaCO3)
- Composition
- Silicon dioxide, often with trace impurities that impart color
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