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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. During fossilization, these structures are replaced by silica, often chalcedony or agate, which allows for the preservation of their distinctive patterns. Rugose corals are characterized by their horn-like shape (solitary forms) or colonial structures with radial septa. Tabulate corals are colonial, forming structures with horizontal plates (tabulae) and often lacking septa, exhibiting honeycomb or chain-like patterns. The fossilization process often highlights these internal structures, creating beautiful and unique patterns.
How to Identify
- Color
- Highly variable, depending on the mineral impurities present during silicification. Common colors include shades of gray, brown, white, black, pink, red, yellow, and blue. Often exhibits multiple colors in banded or mottled patterns.
- Luster
- Waxy to dull, sometimes vitreous (glassy) if the silicification is very fine-grained and pure.
- Texture
- Smooth to slightly granular when polished. The surface often reveals the intricate patterns of the original coral structure, such as septa, tabulae, and corallites. Can feel cool and dense to the touch.
- Crystal Form
- Microcrystalline to cryptocrystalline quartz (chalcedony/agate) replacing the original coral structure. No macroscopic crystal forms are typically visible, but microscopic examination reveals fibrous or granular quartz.
- Cleavage
- None, as it is composed of microcrystalline quartz.
- Geological Environment
- Found in marine sedimentary rock sequences, particularly limestones, shales, and sandstones that were deposited in ancient shallow marine environments where coral reefs thrived. Often associated with fossiliferous beds.
Key Facts
- Hardness: 6.5-7 on Mohs scale (due to silicification)
- Specific Gravity: 2.58-2.64 (typical for chalcedony/agate)
- Crystal System: Trigonal (for quartz/chalcedony)
- Color: Highly variable, often patterned
- Luster: Waxy to dull, sometimes vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2), replacing original calcium carbonate (CaCO3)
Quick Check
- Color: Variable (gray, brown, white, black, pink, red, yellow, blue)
- Luster: Waxy to dull, sometimes vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline to microcrystalline aggregates, pseudomorphous after coral skeletons.
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy, dull, or vitreous
Formation
Fossilized coral forms when the original calcium carbonate (aragonite or calcite) skeletal structures of ancient corals are replaced by silica (chalcedony or agate) through a process of permineralization or replacement. This typically occurs in marine sedimentary environments where silica-rich groundwater percolates through buried coral reefs. The silica preserves the intricate internal and external structures of the coral polyps and colonies.
Usage
Primarily used as an ornamental stone, for lapidary work (cabochons, beads, carvings), and as a decorative material. It is also highly valued by paleontologists and collectors for its scientific and aesthetic appeal.
Age Distribution
Rugosa: Middle Ordovician to Late Permian (approx. 470 to 252 million years ago). Tabulata: Early Ordovician to Late Permian (approx. 485 to 252 million years ago).
Where to Find
Florida, USA
Known for its agatized coral, particularly the 'Florida Agatized Coral' which is the state stone. Found in Miocene-Pliocene deposits.
Indonesia (Sumatra)
Produces beautifully preserved and highly colorful fossilized corals, often from Tertiary deposits.
Morocco
Known for various fossilized marine organisms, including corals, from Paleozoic and Mesozoic strata.
United Kingdom
Various localities, particularly in Carboniferous limestone formations, yield rugose and tabulate corals.
Australia
Significant occurrences in various states, including Western Australia and Queensland, from Paleozoic and Mesozoic marine sequences.
Canada
Found in Paleozoic sedimentary rocks, particularly in Ontario and Quebec.
Finding Tips
Look in Sedimentary Rocks
Focus your search on areas with exposed marine sedimentary rocks, especially limestones, shales, and sandstones that are known to be fossiliferous.
Identify Coral Reef Environments
Research geological maps and literature to identify ancient reef complexes or shallow marine depositional environments from the Ordovician to Permian periods.
Examine Weathered Surfaces
Fossilized corals often weather out of softer matrix rocks. Look for distinctive patterns on exposed rock surfaces or in stream beds and gravels.
Acid Test (Caution)
While the original coral was calcium carbonate, fossilized coral is typically silicified. A drop of dilute hydrochloric acid will not react with silicified coral, but will effervesce on un-fossilized or partially fossilized calcareous coral. Use with extreme caution and only on a small, inconspicuous area.
Similar Rocks
Modern Coral
Scleractinia (Stony Corals)
Also known as: Living Coral, Reef Coral
Petrified Wood
Silicified Wood
Also known as: Fossilized Wood
Chert
Microcrystalline Quartz
Also known as: Flint
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz)
- Group
- Quartz Group (Chalcedony/Agate)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (replacing original CaCO3)
- Composition
- Silicon dioxide, often with trace impurities that impart color.
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