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Fossilized coral is a type of chalcedony that has replaced the original calcium carbonate structure of ancient corals. It exhibits a wide range of colors, often displaying the intricate patterns of the original coral polyps, such as radial septa, growth rings, and cellular structures. The silicification process preserves these details, making each piece unique. It is a durable material, suitable for polishing and cutting.
How to Identify
- Color
- Highly variable, including white, gray, brown, black, pink, red, yellow, and blue. Often multi-colored or banded.
- Luster
- Waxy to dull, sometimes vitreous when polished.
- Texture
- Smooth when polished; rough to slightly granular on unpolished surfaces. The defining characteristic is the visible fossilized coral structure.
- Crystal Form
- Microcrystalline, forming botryoidal, mammillary, or stalactitic masses, but in fossilized coral, it takes the form of the original coral structure.
- Cleavage
- None, due to its microcrystalline nature.
- Geological Environment
- Found in sedimentary rock formations where ancient coral reefs once thrived, and subsequent silicification occurred. Often associated with limestone, chert, and other silica-rich sedimentary deposits.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to chalcedony)
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (for chalcedony, microcrystalline)
- Color: White, gray, brown, black, pink, red, yellow, blue; often multi-colored
- Luster: Waxy to dull, sometimes vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Silicon dioxide (SiO2), with trace impurities that cause color variations
Quick Check
- Color: Variable, often with visible coral patterns
- Luster: Waxy to dull
- Streak: White
Physical Characteristics
- Crystal Habit: Microcrystalline aggregates, pseudomorphous after coral structures.
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy to dull
Formation
Fossilized coral forms when ancient corals, originally composed of calcium carbonate (aragonite or calcite), are gradually replaced by silica (SiO2) through a process called permineralization or replacement. This occurs when silica-rich groundwater percolates through the porous coral structures, dissolving the original carbonate material and simultaneously depositing microcrystalline quartz (chalcedony) in its place. The intricate internal structures of the coral, such as septa and corallites, are often perfectly preserved, creating distinctive patterns.
Usage
Primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, and decorative objects. It is also collected by fossil and mineral enthusiasts. Historically, some cultures may have used it for tools or ceremonial items, though its primary modern use is aesthetic.
Age Distribution
Ranges from Paleozoic to Cenozoic, depending on the coral species and geological setting. Many commercially available specimens are from the Oligocene to Miocene epochs.
Where to Find
Florida, USA
Known for its 'Agatized Coral' (specifically 'Florida Agatized Coral'), which is the state stone. Found in Oligocene-Miocene limestones, particularly in the Tampa Bay area and along the Suwannee River.
Indonesia (Sumatra, Java)
Produces a wide variety of fossilized corals, often with vibrant colors and well-preserved structures, from Cenozoic marine deposits.
Morocco
Known for Paleozoic fossil corals, often found in limestone formations.
Utah, USA
Some occurrences of silicified corals, though less common than Florida.
Australia
Various locations yield fossil corals, including silicified forms, from different geological periods.
Finding Tips
Look for Coral Reef Environments
Search in areas that were once ancient shallow marine environments, particularly those with limestone or chert deposits, as these are conducive to coral growth and subsequent silicification.
Examine Sedimentary Outcrops
Inspect exposed sedimentary rock layers, especially those that appear to be chert or silicified limestone. Look for distinctive patterns indicative of coral structures.
Check Riverbeds and Gravel Pits
Erosion can expose and transport fossilized coral. Riverbeds, gravel pits, and beach deposits in relevant geological areas can be good places to find tumbled specimens.
Acid Test (Caution)
A small drop of dilute hydrochloric acid will not react with silicified coral (chalcedony), whereas un-silicified coral (calcium carbonate) will effervesce. Use extreme caution with acids and wear appropriate safety gear.
Similar Rocks
Agate
Chalcedony (banded or inclusion-rich)
Also known as: Banded Agate, Moss Agate
Jasper
Chalcedony (opaque, iron-rich)
Also known as: Picture Jasper, Red Jasper
Petrified Wood
Chalcedony (silicified wood)
Also known as: Fossil Wood
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal (microcrystalline)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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