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Fossil coral is a type of fossilized marine organism, specifically the skeletal remains of ancient corals (Anthozoa), that has undergone a process of mineralization, typically by silica. This replacement process preserves the original intricate structures of the coral polyps, often displaying distinctive patterns such as radial septa, corallites, and growth rings. The resulting material is typically a form of chalcedony or agate, exhibiting a wide range of natural colors and patterns depending on the original coral species and the mineral impurities present during fossilization.
How to Identify
- Color
- Highly variable, ranging from white, gray, brown, black, yellow, orange, red, pink, to blue, often with multiple colors and patterns within a single specimen. Colors are influenced by mineral impurities during silicification.
- Luster
- Waxy to vitreous (glassy) when polished, dull to earthy in its raw, unpolished state.
- Texture
- Smooth and often translucent when polished; rough and porous in its natural state. The surface may reveal the preserved cellular or skeletal structures of the original coral.
- Crystal Form
- Cryptocrystalline, meaning the individual quartz crystals are too small to be seen without high magnification. The macroscopic form retains the original coral morphology.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in ancient marine sedimentary deposits, particularly in areas that were once shallow, warm seas conducive to coral reef growth. These deposits are often associated with limestone, shale, and sandstone formations, and may be found in uplifted marine terraces or riverbeds that have eroded through such formations.
Key Facts
- Hardness: 6.5 - 7 on the Mohs scale (due to silicification)
- Specific Gravity: 2.58 - 2.64
- Crystal System: Trigonal (for the constituent quartz/chalcedony)
- Color: Highly variable, often multi-colored with distinct patterns
- 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, yellow, orange, red, pink, blue)
- Luster: Waxy to vitreous (polished), dull to earthy (raw)
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, preserving the original biogenic structure of the coral.
- Cleavage Type: Absent
- Fracture Type: Conchoidal, typical of cryptocrystalline quartz.
- Tenacity: Brittle
- Luster Type: Waxy to vitreous
Formation
Fossil coral forms when ancient corals, primarily Anthozoa (which include stony corals), die and their calcium carbonate skeletons are gradually replaced by silica (chalcedony or agate) through a process called permineralization or replacement. This occurs when silica-rich groundwater percolates through the porous coral structures, depositing microscopic quartz crystals that slowly replace the original organic material and calcium carbonate, preserving the intricate patterns of the coral polyps. The process can take millions of years and requires specific geochemical conditions, including the presence of dissolved silica and a relatively stable burial environment.
Usage
Fossil coral is primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, decorative objects, and polished display pieces. Its unique patterns and colors make it popular in lapidary arts. It is also collected by paleontologists and amateur fossil enthusiasts for scientific study and display.
Age Distribution
Fossil corals can range in age from the Ordovician Period (approximately 485 million years ago) to the Pliocene Epoch (approximately 2.58 million years ago), with significant occurrences in the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Cenozoic periods.
Where to Find
Florida, USA
Known for its 'agatized coral' from the Tampa Bay area, particularly the Withlacoochee River and other central Florida locations. These are often from the Oligocene and Miocene epochs.
Indonesia (especially Sumatra)
A significant source of high-quality fossil coral, often exhibiting vibrant colors and intricate patterns. These are typically from Cenozoic marine deposits.
Morocco
Produces fossil corals, often found in limestone formations, dating back to various Paleozoic and Mesozoic periods.
Utah, USA
Some fossil coral occurrences, though less common than petrified wood, can be found in marine sedimentary layers.
Australia
Various locations yield fossil corals, particularly from ancient reef systems.
United Kingdom
Devonian and Carboniferous fossil corals are found in certain limestone formations.
Finding Tips
Geological Maps
Consult geological maps to identify areas with ancient marine sedimentary rock formations, especially those known for coral reefs or shallow marine environments.
Riverbeds and Beaches
Look in riverbeds, stream banks, and beaches in areas known for fossil coral, as erosion can expose and transport specimens. Florida's agatized coral is often found in these environments.
Quarries and Road Cuts
Exposed rock faces in quarries, road cuts, and construction sites can reveal fossil-bearing layers. Always obtain permission before collecting on private land or in active work sites.
Distinguishing from Modern Coral
Fossil coral is typically much denser and harder than modern coral due to silicification. It will not react with acid (like modern calcium carbonate coral would) and will often have a waxy or glassy luster when broken or polished.
Look for Patterns
The key identifying feature is the preserved internal structure of the coral polyps, which can appear as radial patterns, honeycomb-like cells, or branching forms within the stone.
Similar Rocks
Petrified Wood
Silicified Wood
Also known as: Fossilized Wood
Agate
SiO2
Also known as: Chalcedony
Modern Coral
Corallium rubrum (for precious coral)
Also known as: Precious Coral, Red Coral
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz/chalcedony)
- Group
- Quartz group (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (after replacement)
- Composition
- Silicon dioxide, with trace impurities that contribute to color variations. The original coral was primarily calcium carbonate (aragonite or calcite).
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