Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Fossilized coral is a type of fossil where the skeletal remains of ancient corals have been preserved, typically through silicification. The original coral, which could be either Scleractinian (stony corals, often reef-building) or Tabulate (an extinct group of colonial corals), is replaced by silica, forming 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 are characteristic of the original coral species. The color can vary widely depending on the trace minerals present during silicification.
How to Identify
- Color
- Highly variable, ranging from white, gray, brown, black, pink, red, orange, yellow, to blue, often with multiple colors in a single specimen. The color is influenced by impurities (e.g., iron oxides for reds/browns, manganese for blacks).
- Luster
- Vitreous (glassy) to waxy, sometimes dull.
- Texture
- Smooth to slightly granular when polished; unpolished surfaces can be rough. The most distinctive feature is the preserved internal structure of the coral, appearing as intricate patterns of circles, polygons, or radiating lines, representing the corallites, septa, and tabulae.
- Crystal Form
- Cryptocrystalline (microscopic crystals), typically chalcedony, which is a fibrous variety of quartz. No macroscopic crystal forms are visible.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in marine sedimentary rock formations, particularly limestones, shales, and sandstones, that were deposited in ancient shallow, warm seas where coral reefs flourished. Often associated with other marine fossils.
Key Facts
- Hardness: 6.5-7 (Mohs scale, due to silicification)
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (for quartz/chalcedony component)
- Color: Highly variable, often banded or mottled with preserved coral patterns
- Luster: Vitreous to waxy
- Transparency: Translucent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2), with trace impurities causing color variations. Original coral was calcium carbonate (CaCO3).
Quick Check
- Color: Variable (white, gray, brown, black, pink, red, orange, yellow, blue)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, forming botryoidal, mammillary, or stalactitic masses, often filling voids or replacing organic structures.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Fossilized coral forms when the original calcium carbonate (aragonite or calcite) skeletal structure of ancient corals is replaced by silica (chalcedony or quartz) through a process called permineralization or replacement. This typically occurs in marine sedimentary environments where corals thrived. After the coral dies, its porous skeleton is buried by sediments. Groundwater rich in dissolved silica then infiltrates the skeleton, and over millions of years, the silica precipitates within the pores and gradually replaces the original calcium carbonate, preserving the intricate internal and external structures of the coral polyps and colonies. The resulting material is often a form of chalcedony, a cryptocrystalline variety of quartz.
Usage
Fossilized coral is primarily used as an ornamental stone, particularly in jewelry (cabochons, beads), carvings, and decorative objects. Its unique patterns, which reflect the original coral structures, make it highly prized. It is also collected by paleontologists and hobbyists for its scientific and aesthetic value as a fossil.
Age Distribution
Paleozoic to Cenozoic Eras (approximately 541 million years ago to present), with significant occurrences in the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Cenozoic periods.
Where to Find
Florida, USA
Known for its agatized coral, particularly from the Tampa Bay area, which is the state fossil. These are primarily Cenozoic (Miocene) Scleractinian corals.
Indonesia (especially Sumatra)
Produces a wide variety of beautifully patterned and colored fossilized corals, often from Cenozoic deposits.
Morocco
Known for Devonian and Carboniferous fossil corals, often found in limestone matrices.
Utah, USA
Various localities yield fossil corals, including Paleozoic specimens.
Australia
Diverse fossil coral occurrences across different geological periods.
United Kingdom
Paleozoic coral fossils are found in various limestone formations.
Finding Tips
Identify Sedimentary Environments
Look for areas with ancient marine sedimentary rock outcrops, especially limestones, shales, and sandstones, which indicate past shallow marine environments conducive to coral growth.
Examine Weathered Surfaces
Fossilized coral can be more easily identified on weathered rock surfaces where the surrounding matrix has eroded away, exposing the more resistant silicified coral.
Look for Distinctive Patterns
The most reliable identification feature is the preserved internal structure of the coral polyps, appearing as radial, polygonal, or circular patterns. These are distinct from other rock textures.
Check Hardness
Since it's typically silicified, fossilized coral will be hard (Mohs 6.5-7), scratching glass and steel. This distinguishes it from un-silicified coral fossils (calcite, Mohs 3).
Acid Test (Caution Recommended)
A small drop of dilute hydrochloric acid will not react with silicified coral, whereas un-silicified calcium carbonate coral will effervesce. Use this test sparingly and with caution, as it can damage specimens.
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
- Silicate (specifically Tectosilicate, as quartz/chalcedony)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (after silicification)
- Composition
- Silicon dioxide, often with minor impurities of iron, manganese, and other elements.
Explore Fossilized Coral
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.