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Fossilized coral represents the preserved skeletal remains of ancient marine invertebrates belonging to the class Anthozoa, phylum Cnidaria. The two main extinct orders commonly found as fossils are Rugosa (horn corals) and Tabulata (tabulate corals). Rugose corals were solitary or colonial, characterized by prominent septa (radial partitions) and often a horn-like shape. Tabulate corals were exclusively colonial, forming massive, branching, or encrusting structures with horizontal partitions called tabulae and lacking septa or having very rudimentary ones. During fossilization, the original calcium carbonate skeleton is replaced by silica, resulting in a durable and often beautifully patterned material.
How to Identify
- Color
- Highly variable, depending on the mineral impurities present during silicification. Common colors include white, gray, brown, black, pink, red, yellow, and blue. Often exhibits concentric banding or mottled patterns.
- Luster
- Vitreous (glassy) to waxy, characteristic of chalcedony or quartz.
- Texture
- Smooth when polished, but can be rough or granular on unpolished surfaces. The surface often reveals the intricate patterns of the original coral polyps, septa, or tabulae.
- Crystal Form
- Cryptocrystalline (chalcedony) or microcrystalline (quartz), meaning individual crystals are too small to be seen without magnification. The overall form reflects the original coral morphology.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in marine sedimentary rock formations that were once ancient shallow, warm seas where corals thrived. These include limestones, shales, and sandstones, particularly those associated with ancient reef systems.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification).
- Specific Gravity: 2.58-2.64 (typical for chalcedony/quartz).
- Crystal System: Trigonal (for the replacing quartz/chalcedony).
- Color: Highly variable, often patterned.
- Luster: Vitreous to waxy.
- Transparency: Translucent to opaque.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2) as chalcedony or quartz, replacing original calcium carbonate (CaCO3).
Quick Check
- Color: Variable (white, gray, brown, black, pink, red, yellow, blue), often patterned.
- Luster: Vitreous to waxy.
- Streak: White (characteristic of chalcedony/quartz).
Physical Characteristics
- Crystal Habit: Cryptocrystalline or microcrystalline aggregates, preserving the original biogenic structure of the coral.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to waxy.
Formation
Fossilized coral forms when ancient coral skeletons, primarily composed of calcium carbonate (aragonite or calcite), are replaced by silica (chalcedony or quartz) through a process called permineralization or replacement. This typically occurs in marine sedimentary environments where corals lived, died, and were subsequently buried by sediments. Groundwater rich in dissolved silica then infiltrates the porous coral structure, slowly replacing the original calcium carbonate with silica while preserving the intricate details of the coral's internal and external morphology.
Usage
Fossilized coral is primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, and decorative objects. Its unique patterns and colors make it popular in lapidary arts. It is also collected by paleontologists and amateur fossil enthusiasts for its scientific and aesthetic value.
Age Distribution
Ordovician to Permian (Rugosa), Ordovician to Permian (Tabulata). Modern corals (Scleractinia) appeared in the Middle Triassic.
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 large quantities of beautifully patterned fossilized coral, often with vibrant colors.
Morocco
Known for Devonian-age rugose and tabulate corals, often preserved in limestone.
United Kingdom (e.g., Wenlock Edge, Shropshire)
Silurian-age tabulate and rugose corals are abundant in limestone formations.
Australia (e.g., Queensland)
Various localities yield fossilized corals from different geological periods.
USA (various states)
Found in numerous states with ancient marine sedimentary rocks, including Michigan (Petoskey Stone - a colonial rugose coral, Hexagonaria percarinata), Indiana, Kentucky, and New York.
Finding Tips
Research Local Geology
Identify areas known for marine sedimentary rocks from the Ordovician to Permian periods. Geological maps and local fossil clubs are excellent resources.
Look in Riverbeds and Gravel Pits
Erosion can expose fossilized coral in riverbeds, stream banks, and gravel pits, especially in regions with underlying marine sedimentary bedrock.
Examine Limestone Outcrops
Many fossil corals are preserved within limestone. Look for weathered surfaces where the coral structure might be more visible.
Check for Distinctive Patterns
Look for the characteristic radial septa (rugose) or honeycomb/chain-like patterns (tabulate) on weathered rock surfaces. Polished specimens will show these patterns more clearly.
Acid Test (Caution)
A small drop of dilute hydrochloric acid (HCl) will not react with silicified coral, distinguishing it from un-silicified calcium carbonate coral which will effervesce. Use extreme caution with acids and wear appropriate safety gear.
Similar Rocks
Petrified Wood
Silicified plant material
Also known as: Silicified Wood
Agate
SiO2
Also known as: Chalcedony
Modern Coral
Scleractinia
Also known as: Scleractinian Coral
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz/chalcedony)
- Group
- Quartz group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (replacing original CaCO3)
- Composition
- Silicon dioxide (SiO2) with trace impurities.
Explore Fossilized Coral (Rugose or Tabulate Coral)
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