Fossilized Coral

Description

Fossilized coral, specifically Rugose corals (often called 'Horn Corals' due to their typically solitary, horn-shaped growth form), are ancient marine invertebrates. These fossils are commonly found embedded within sedimentary rocks, most notably limestone and chert. The preservation quality varies, from simple external molds to detailed internal structures. When preserved in chert, the silicification process often highlights the septa (radial partitions) and tabulae (horizontal plates) of the coral, making them highly visible and aesthetically pleasing when cut and polished. In limestone, the coral structure may be less distinct or recrystallized.

How to Identify

Color

Matrix color varies widely: Limestone can be white, gray, tan, brown, or black. Chert can be white, gray, black, brown, red, or green. The fossilized coral itself often appears as a lighter or darker contrast to the matrix, or can be the same color if completely replaced by the matrix material. Silicified corals often show a translucent to opaque appearance.

Luster

Matrix: Limestone is typically dull to earthy. Chert is typically waxy to dull, sometimes vitreous (glassy) on fresh fracture. The fossilized coral structure within chert can exhibit a waxy to vitreous luster when polished.

Texture

Matrix: Limestone can be fine-grained to coarse-grained, often with a gritty or smooth feel. Chert is typically very fine-grained (cryptocrystalline) and smooth to the touch, with a conchoidal fracture. The fossil itself will show the characteristic morphology of the coral, such as radial septa and concentric growth lines, which can be felt as subtle ridges or seen as patterns.

Crystal Form

The fossil itself retains the original biological form of the Rugose coral, which is typically solitary and horn-shaped (conical to cylindrical), or less commonly colonial. The internal structure includes radial septa and horizontal tabulae. The matrix (chert or limestone) is cryptocrystalline (chert) or microcrystalline to macrocrystalline (limestone) and does not exhibit distinct crystal forms unless it's a large calcite crystal.

Cleavage

Limestone (calcite) exhibits perfect rhombohedral cleavage (3 directions not at 90 degrees). Chert has no cleavage, exhibiting conchoidal fracture. The fossilized coral structure itself does not exhibit cleavage, but rather fracture along its preserved biological structures.

Geological Environment

Marine sedimentary environments, particularly shallow, warm seas where coral reefs or isolated coral growth flourished. Found in ancient marine limestones, shales, and chert beds that formed from the diagenesis of silica-rich sediments or replacement of carbonate sediments.

Key Facts

• Hardness: 3 (for calcite in limestone) to 7 (for quartz in chert) on the Mohs scale. The fossil itself will have the hardness of the material it is composed of or replaced by.
• Specific Gravity: 2.6-2.8 (for calcite/limestone) to 2.6-2.7 (for quartz/chert).
• Crystal System: Trigonal (for calcite) or Hexagonal (for quartz). The fossil retains the biological form, not a mineral crystal system.
• Color: Highly variable, depending on the matrix and diagenetic alteration. Common colors include grays, browns, whites, and blacks.
• Luster: Dull to waxy (chert), earthy (limestone), waxy to vitreous (polished silicified coral).
• Transparency: Opaque to translucent (especially silicified specimens).
• Fracture: Conchoidal (chert), uneven to splintery (limestone). The fossil itself will fracture according to its preserved structure and composition.
• Cleavage: Perfect rhombohedral (calcite in limestone), none (chert).
• Composition: Primarily Calcium Carbonate (CaCO3) in limestone, or Silicon Dioxide (SiO2) when silicified in chert. Trace elements can vary depending on the original organism and diagenetic fluids.

Quick Check

• Color: Variable (matrix dependent), often contrasting with matrix
• Luster: Dull to waxy (chert), earthy (limestone), waxy to vitreous (polished silicified coral)
• Streak: White (for both chert and limestone)

Physical Characteristics

• Crystal Habit: Not applicable to the fossil itself, which is a biogenic structure. The matrix minerals (calcite or quartz) have their own habits (e.g., rhombohedral for calcite, microcrystalline for chert).
• Cleavage Type: Perfect rhombohedral (calcite), none (chert).
• Fracture Type: Conchoidal (chert), uneven to splintery (limestone).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, waxy, or vitreous.

Formation

Rugose corals were marine organisms that secreted a calcium carbonate (aragonite or calcite) skeleton. Upon death, these skeletons accumulated on the seafloor. Over geological time, these skeletal remains were buried and underwent diagenesis. In limestone, the original calcium carbonate skeleton may be preserved or recrystallized to calcite. In chert, the original calcium carbonate was replaced by silica (SiO2) through a process called silicification, often from dissolved silica in pore waters, preserving the intricate internal and external structures of the coral.

Usage

Primarily collected as fossils for scientific study, educational purposes, and as decorative items. When silicified and polished, especially varieties like the Petoskey Stone, they are used in jewelry, lapidary art, and as ornamental stones. They serve as important paleoenvironmental indicators.

Age Distribution

Ordovician to Permian periods (approximately 485 to 252 million years ago), with peak diversity in the Devonian and Carboniferous.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a single mineral, but a fossil within a rock. The primary mineral components are Carbonates (for limestone) or Silicates (for chert).

Group

Fossilized Invertebrate (Rugosa, an extinct order of corals).

Crystal System

Trigonal (for calcite) or Hexagonal (for quartz).

Chemical Formula

CaCO3 (if preserved as calcite) or SiO2 (if silicified).

Composition

Calcium carbonate (calcite) or silicon dioxide (quartz/chert), preserving the skeletal structure of an ancient Rugose coral.

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