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Fossilized coral is a type of fossil where the skeletal remains of ancient corals (Anthozoa) have been preserved, typically through silicification. The original calcium carbonate structure is replaced by cryptocrystalline quartz (chalcedony), resulting in a durable and often beautifully patterned material. The patterns observed in fossilized coral are the preserved internal structures of the coral polyps, such as septa, tabulae, and corallites, which can appear as intricate floral or cellular designs. The color varies widely depending on the mineral impurities present during silicification.
How to Identify
- Color
- Highly variable, including shades of white, gray, brown, black, pink, red, yellow, and blue. Often exhibits multiple colors in concentric or patterned arrangements.
- Luster
- Waxy to vitreous (glassy) when polished, dull to earthy when unpolished.
- Texture
- Smooth to slightly granular when polished. Unpolished surfaces may retain some original coral texture. Internal patterns are characteristic.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without magnification. The overall form reflects the original coral morphology (e.g., massive, branching, colonial).
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in sedimentary rock formations that were once ancient marine environments, particularly shallow, warm seas where coral reefs thrived. Often associated with limestone, shale, and sandstone deposits, especially those that have undergone silicification.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification).
- Specific Gravity: 2.58-2.64 (typical for chalcedony/quartz).
- Crystal System: Trigonal (for the constituent quartz/chalcedony).
- Color: Highly variable, often exhibiting patterns of the original coral structure.
- Luster: Waxy to vitreous.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2), replacing original calcium carbonate (CaCO3).
Quick Check
- Color: Variable (white, gray, brown, black, pink, red, yellow, blue), often patterned.
- Luster: Waxy to vitreous (polished), dull to earthy (unpolished).
- Streak: White (due to its chalcedony/quartz composition).
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, preserving the original biogenic structure of the coral.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal, typical of quartz and chalcedony.
- Tenacity: Brittle.
- Luster Type: Waxy to vitreous.
Formation
Fossilized coral forms when ancient coral polyps, which secrete calcium carbonate (aragonite or calcite) to build their skeletons, die and are subsequently buried by sediments. Over geological time, the original calcium carbonate skeleton is replaced by silica (chalcedony or quartz) through a process called permineralization or replacement. This silicification preserves the intricate internal and external structures of the coral, often in remarkable detail. The process typically occurs in marine environments where silica-rich waters are present, often associated with volcanic activity or the dissolution of siliceous organisms like diatoms and radiolarians.
Usage
Primarily used as an ornamental stone, for lapidary work (cabochons, beads, carvings), and in jewelry. It is also collected by fossil enthusiasts and used in educational displays. Historically, some cultures may have used it for decorative or ceremonial purposes.
Age Distribution
Ranges from the Cambrian Period (approximately 541 million years ago) to the present, with significant fossilization events occurring in the Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Cenozoic Eras.
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 (especially Sumatra)
Produces a wide variety of beautifully patterned and colored fossilized corals, often from Cenozoic-era marine sediments.
Morocco
Known for Devonian-age fossil corals, often preserved in limestone.
Utah, USA
Some occurrences of fossilized coral, though less common than petrified wood.
Australia
Various locations yield fossil corals from different geological periods.
United Kingdom
Carboniferous and Devonian coral fossils are found in certain limestone formations.
Finding Tips
Geological Maps
Consult geological maps of regions known for ancient marine sedimentary deposits, particularly those indicating limestone or shale formations that might contain fossils.
Coastal and Riverine Exposures
Look in areas where erosion has exposed older sedimentary layers, such as sea cliffs, riverbeds, and gravel pits. Fossilized coral can be found as float in these areas.
Known Fossil Localities
Research specific fossil collecting sites or quarries known for marine fossils. Always obtain permission before collecting on private land.
Examine Rock Matrix
Fossilized coral is often embedded in a matrix of limestone, chert, or sandstone. Look for distinctive coral patterns on weathered surfaces.
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
- Tectosilicate (as quartz/chalcedony)
- Group
- Quartz group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (after silicification)
- Composition
- Silicon dioxide, with trace impurities causing color variations. Original composition was calcium carbonate (aragonite or calcite).
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