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Fossilized coral is a biogenic sedimentary rock formed from the fossilized remains of ancient coral colonies. The original calcium carbonate skeleton of the coral is replaced by silica, typically chalcedony, through a process of permineralization or replacement. This silicification preserves the detailed internal structures, such as septa, tabulae, and corallites, which are characteristic of the original coral species. The resulting material is hard, durable, and often exhibits a wide range of colors and intricate patterns, making it highly prized for decorative and lapidary purposes. The patterns can resemble flowers, stars, or honeycomb, depending on the type of coral and the angle of cut.
How to Identify
- Color
- Highly variable, including shades of white, gray, brown, black, pink, red, yellow, and blue. The color is influenced by trace minerals present during silicification.
- Luster
- Waxy to vitreous (glassy) when polished, dull to earthy in its natural state.
- Texture
- Microcrystalline, dense, and smooth when polished. Unpolished specimens may show a slightly rougher texture. The defining characteristic is the preserved internal structure of the coral, visible as distinct patterns.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. It forms massive, botryoidal, or stalactitic aggregates, but in fossilized coral, it takes the form of the original coral skeleton.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in ancient marine sedimentary rock formations, particularly limestones, shales, and sandstones, where coral reefs once thrived. These environments indicate past shallow, warm, clear marine waters.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification)
- Specific Gravity: 2.58-2.64 (typical for chalcedony)
- Crystal System: Trigonal (for the constituent chalcedony/quartz)
- Color: Highly variable, often exhibiting natural patterns of the original coral structure.
- Luster: Waxy to vitreous when polished, dull to earthy in rough form.
- 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)
- Luster: Waxy to vitreous (polished), dull to earthy (unpolished)
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, forming the preserved structure of the original coral colony.
- Cleavage Type: None.
- Fracture Type: Conchoidal, producing smooth, curved surfaces.
- Tenacity: Brittle.
- Luster Type: Waxy to vitreous.
Formation
Fossilized coral forms when ancient coral polyps die, leaving behind their calcium carbonate (aragonite or calcite) skeletons. Over geological time, these skeletons are buried by sediments. Groundwater rich in dissolved silica (SiO2) then percolates through the porous coral structures. The silica gradually replaces the original calcium carbonate, molecule by molecule, in a process called permineralization or replacement. This silicification preserves the intricate internal and external structures of the coral, often transforming the original aragonite/calcite into chalcedony (a microcrystalline variety of quartz). The resulting fossil is highly durable and retains the characteristic patterns of the original coral colony.
Usage
Fossilized coral is primarily used as an ornamental stone, particularly in lapidary arts for cabochons, beads, carvings, and polished specimens. Its unique patterns and colors make it popular in jewelry and decorative objects. It is also collected by paleontologists and fossil enthusiasts for its scientific and aesthetic value. Historically, some cultures may have used it for medicinal or spiritual purposes, though these are not scientifically validated.
Age Distribution
Ranges from Ordovician (approximately 485 million years ago) to Neogene (approximately 2.6 million years ago), with significant occurrences in the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Paleogene periods.
Where to Find
Florida, USA
Known for its 'Agatized Coral' (specifically 'Florida Agatized Coral'), which is the state stone. These fossils are primarily from the Oligocene and Miocene epochs, found in limestone deposits.
Indonesia (especially Sumatra)
Produces a wide variety of beautifully patterned and colored fossilized corals, often from Tertiary period marine deposits.
Morocco
Known for Devonian-age fossilized corals, often found in limestone and shale formations.
Utah, USA
Various locations yield fossilized corals, particularly from Paleozoic marine sediments.
Australia
Diverse occurrences across different states, including Western Australia and Queensland, from various geological periods.
China
Significant deposits of fossilized corals, often from Paleozoic and Mesozoic eras.
Finding Tips
Research Geological Maps
Consult geological maps of areas known for ancient marine sedimentary rocks, especially those indicating past reef environments or limestone formations. Look for areas with exposed bedrock or eroded sedimentary layers.
Look for Associated Fossils
Fossilized coral is often found alongside other marine fossils such as brachiopods, crinoids, and mollusks, indicating a marine sedimentary environment.
Examine Outcrops and Riverbeds
Search in exposed rock outcrops, road cuts, quarries, and riverbeds where erosion has uncovered older sedimentary layers. Weathering can sometimes reveal the patterns more clearly.
Check for Distinctive Patterns
Look for rock fragments exhibiting the characteristic radial, honeycomb, or star-like patterns of coral polyps. These patterns are often more visible on weathered surfaces or when the rock is wet.
Test Hardness
Since it's silicified, fossilized coral will be harder than typical limestone (calcite). It should scratch glass and not be easily scratched by a steel knife (Mohs hardness 6.5-7).
Similar Rocks
Petrified Wood
Silicified Wood
Also known as: Fossilized Wood
Agate
Cryptocrystalline Quartz
Also known as: Chalcedony
Chert
Cryptocrystalline Quartz
Also known as: Flint
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as chalcedony/quartz)
- Group
- Quartz group (cryptocrystalline variety)
- Crystal System
- Trigonal (for the constituent chalcedony/quartz)
- Chemical Formula
- SiO2 (replacing original CaCO3)
- Composition
- Silicon dioxide, with trace impurities that impart color.
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