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Fossilized coral is a type of fossil where the skeletal remains of ancient corals have been replaced by minerals, most commonly silica (chalcedony or agate). This replacement process preserves the detailed internal structures of the coral, such as the septa (radial partitions) and tabulae (horizontal plates), which are characteristic of different coral species. The resulting material is typically hard, durable, and often exhibits a wide range of colors and patterns, depending on the original coral species and the mineralizing fluids. Hexagonaria percarinata, commonly known as Petoskey Stone, is a well-known example of a fossilized tabulate coral, characterized by its distinctive hexagonal patterns representing individual coral polyps.
How to Identify
- Color
- Highly variable, ranging from white, gray, brown, black, pink, red, yellow, to blue, often with contrasting patterns. The color is influenced by trace minerals present during silicification.
- Luster
- Waxy to vitreous (glassy) when polished, dull to earthy on unpolished or weathered surfaces.
- Texture
- Smooth and dense when polished. Unpolished surfaces may feel slightly granular or rough. The defining characteristic is the visible internal structure of the coral, such as hexagonal patterns (Hexagonaria), radiating septa, or concentric growth rings.
- Crystal Form
- Cryptocrystalline (microscopic crystals of quartz), forming a massive, dense aggregate that preserves the original coral morphology.
- Cleavage
- None, due to its cryptocrystalline nature. It exhibits conchoidal fracture.
- Geological Environment
- Found in ancient marine sedimentary rock formations that once hosted coral reefs. These environments are typically shallow, warm, clear marine waters. After burial, groundwater rich in dissolved silica infiltrates the coral skeletons, leading to their replacement.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification).
- Specific Gravity: 2.58-2.64 (typical for chalcedony/agate).
- Crystal System: Trigonal (for the constituent quartz/chalcedony).
- Color: Highly variable, often with distinct patterns of the original coral structure.
- 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, pink, red, yellow, blue), often patterned.
- Luster: Waxy to vitreous (polished), dull to earthy (unpolished).
- Streak: White.
Physical Characteristics
- Crystal Habit: Cryptocrystalline, massive, preserving the original biogenic structure of the coral.
- Cleavage Type: None.
- Fracture Type: Conchoidal, typical of quartz.
- Tenacity: Brittle.
- Luster Type: Waxy to vitreous.
Formation
Fossilized coral forms when ancient coral reefs are buried by sediments. Over geological time, the original calcium carbonate (aragonite or calcite) of the coral skeleton is replaced by silica (chalcedony or agate) through a process called permineralization or replacement. This silicification preserves the intricate internal and external structures of the coral polyps and colonies. The replacement by silica makes the fossil much harder and more durable than the original coral skeleton.
Usage
Primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, decorative objects, and polished display specimens. Its unique patterns and colors make it highly sought after by collectors and artisans. Historically, some cultures may have used it for tools or ceremonial objects, though its primary modern use is aesthetic.
Age Distribution
Paleozoic to Cenozoic Eras, depending on the coral type. Hexagonaria percarinata is specifically Devonian (approximately 380-360 million years ago).
Where to Find
Michigan, USA
Famous for Petoskey Stones (Hexagonaria percarinata) found in Devonian-age limestones and glacial till, particularly along the shores of Lake Michigan and Lake Huron.
Florida, USA
Known for agatized coral, particularly from the Tampa Bay area, often exhibiting intricate patterns and a range of colors.
Indonesia
Significant source of fossilized coral, often with vibrant colors and diverse coral species, found in various sedimentary deposits.
Morocco
Produces fossilized corals, often embedded in limestone matrices, from various Paleozoic and Mesozoic formations.
Australia
Various locations yield fossilized corals from different geological periods, including silicified specimens.
United Kingdom
Certain Carboniferous and Devonian limestones contain fossilized corals, though silicified specimens suitable for lapidary work are less common than in other regions.
Finding Tips
Look in Sedimentary Rocks
Focus on areas with ancient marine sedimentary rock outcrops, especially limestones, shales, and sandstones that were deposited in shallow marine environments.
Search Glacial Till
In regions like Michigan, glacial deposits (till) can contain eroded and transported fossilized corals from underlying bedrock.
Examine Riverbeds and Beaches
Water erosion can expose and concentrate fossilized corals in riverbeds, stream banks, and along lake and ocean beaches.
Identify Coral Structures
Look for characteristic coral patterns: hexagonal cells (Hexagonaria), radiating septa, or other colonial growth forms. Weathered surfaces might reveal these patterns more clearly.
Test Hardness
Silicified coral will be hard (Mohs 6.5-7), scratching glass or steel. This distinguishes it from un-silicified, softer calcium carbonate coral fossils.
Check for Luster
Polished or wet specimens will often show a waxy to vitreous luster, enhancing the visibility of the internal patterns.
Similar Rocks
Chert
Microcrystalline Quartz
Also known as: Flint, Jasper
Agate
Cryptocrystalline Quartz
Also known as: Banded Chalcedony
Petrified Wood
Silicified Wood
Also known as: Fossil Wood
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate, as quartz/chalcedony)
- Group
- Quartz group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2 (after silicification)
- Composition
- Silicon dioxide, with trace impurities causing color variations. The original coral was calcium carbonate (CaCO3).
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