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Fossilized coral is a type of fossil where the skeletal remains of ancient corals have been preserved, often through silicification. The resulting material typically exhibits the characteristic patterns of the original coral colonies, such as radial septa, corallites, and growth rings. The color and pattern vary widely depending on the original coral species, the minerals present during fossilization, and subsequent geological processes. It is highly prized for its aesthetic appeal and unique patterns.
How to Identify
- Color
- Highly variable, ranging from white, gray, brown, black, pink, red, yellow, to blue. Often exhibits multiple colors in intricate patterns.
- Luster
- Vitreous to waxy, sometimes dull if unpolished.
- Texture
- Typically smooth when polished, but can be rough or granular in its natural state. The surface often reveals the intricate patterns of the original coral structure.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. The overall form is dictated by the original coral morphology (e.g., massive, branching, columnar, tabulate, rugose).
- Cleavage
- None, due to its cryptocrystalline nature.
- Geological Environment
- Found in sedimentary rock formations that were once ancient marine environments, particularly shallow, warm seas where coral reefs thrived. These include limestones, shales, and sandstones, often in areas that have undergone silicification.
Key Facts
- Hardness: 6.5 - 7 on the Mohs scale (for silicified coral)
- Specific Gravity: 2.58 - 2.64 (for silicified coral)
- Crystal System: Trigonal (for chalcedony replacement)
- Color: Wide range, often patterned
- Luster: Vitreous to waxy
- Transparency: Opaque to translucent
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2) when silicified; originally calcium carbonate (CaCO3)
Quick Check
- Color: Variable, often with visible coral patterns
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, preserving original coral morphology
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Fossilized coral forms when ancient corals, which are marine invertebrates (phylum Cnidaria, class Anthozoa), die and their calcium carbonate (aragonite or calcite) skeletons are preserved through a process of permineralization or replacement. In many cases, particularly for the material commonly used in lapidary arts, the original calcium carbonate is replaced by silica (chalcedony, a cryptocrystalline variety of quartz). This replacement occurs when silica-rich groundwater percolates through the porous coral structure, dissolving the original carbonate and simultaneously depositing silica in its place, preserving the intricate details of the coral's polyps and skeletal structure. This process can take millions of years.
Usage
Primarily used as an ornamental stone, for carvings, cabochons, beads, and other lapidary purposes. It is also of significant scientific interest for paleontological and geological studies, providing insights into ancient marine environments, climate, and biodiversity. Some cultures attribute metaphysical properties to fossilized coral.
Age Distribution
Paleozoic to Cenozoic, with significant occurrences in Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Tertiary periods.
Where to Find
Florida, USA
Known for its 'Agatized Coral' (Florida State Stone), particularly from the Tampa Bay area, found in Miocene-Pliocene age formations.
Indonesia (Sumatra)
Produces a wide variety of beautifully patterned fossilized corals, often from Tertiary age deposits.
Morocco
Known for Devonian age rugose and tabulate corals, often preserved in limestone.
Utah, USA
Various Paleozoic and Mesozoic formations contain fossilized corals.
Australia
Diverse occurrences across different states, including silicified corals.
United Kingdom
Carboniferous limestones contain numerous fossil coral species.
Finding Tips
Geological Maps
Consult geological maps for areas known to have ancient marine sedimentary rock formations, especially those from periods when coral reefs were abundant (e.g., Devonian, Carboniferous, Permian, Mesozoic, Cenozoic).
Coastal and River Exposures
Look in areas where sedimentary layers are exposed, such as coastal cliffs, riverbeds, quarries, and road cuts. Erosion often reveals fossils.
Limestone and Chert Deposits
Fossilized corals are frequently found in association with limestone, chert, and silicified zones within sedimentary sequences.
Paleontological Sites
Research known paleontological sites or fossil collecting localities in your region. Many areas are famous for their coral fossils.
Surface Collection
In some areas, fossilized coral can be found as 'float' on the surface, weathered out of the surrounding rock.
Similar Rocks
Petrified Wood
Wood (plant material) replaced by Chalcedony (SiO2)
Also known as: Agatized Wood, Silicified Wood
Dendritic Agate
Chalcedony (SiO2) with manganese or iron oxide inclusions
Also known as: Tree Agate
Turritella Agate
Chalcedony (SiO2) with fossilized gastropods (Elimia tenera)
Also known as: Elimia Agate
Scientific Classification
- Mineral Class
- Silicate (for silicified coral); Carbonate (for original coral skeleton)
- Group
- Quartz group (for silicified coral)
- Crystal System
- Trigonal (for chalcedony)
- Chemical Formula
- SiO2 (for silicified coral); CaCO3 (for original coral skeleton)
- Composition
- Silicon dioxide (chalcedony) replacing calcium carbonate (aragonite/calcite)
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