Fossilized Coral (Tabulate Coral)

Description

Fossilized coral represents the preserved skeletal remains of ancient coral colonies. Tabulate corals, such as Favosites, are characterized by their colonial growth form, typically massive, hemispherical, or branching, composed of numerous small, polygonal corallites (individual coral polyps' skeletons) packed closely together. The defining feature of tabulate corals is the presence of horizontal partitions called 'tabulae' within the corallites, which represent successive growth stages of the polyp. Septa (vertical partitions) are typically absent or rudimentary in tabulate corals. When fossilized, the original calcium carbonate is often replaced by cryptocrystalline quartz (chalcedony or chert), preserving the detailed internal structure, including the polygonal outlines of the corallites and the tabulae. The color varies widely depending on the mineral impurities introduced during fossilization.

How to Identify

Color

Highly variable, ranging from white, gray, brown, black, pink, red, yellow, to blue, often with contrasting patterns due to mineral impurities (e.g., iron oxides for reds/browns, manganese for blacks).

Luster

Vitreous to waxy, depending on the degree of silicification and polish.

Texture

Smooth when polished; rough to granular on unpolished surfaces. The characteristic texture is the visible pattern of individual corallites, often polygonal (hexagonal in Favosites) or circular, with internal structures like tabulae or septa visible.

Crystal Form

Cryptocrystalline (chalcedony/chert) or microcrystalline calcite, preserving the original biogenic structure of the coral colony. Not a true crystal form in the mineralogical sense, but rather a pseudomorph after the coral skeleton.

Cleavage

None, as it is typically composed of cryptocrystalline quartz or microcrystalline calcite. Fracture is conchoidal to uneven.

Geological Environment

Found in marine sedimentary rock formations, particularly limestones, shales, and cherts, that were deposited in ancient shallow, warm, clear marine environments conducive to coral growth. Often associated with ancient reef complexes.

Key Facts

• Hardness: 6.5-7 on Mohs scale (if silicified as chalcedony/chert); 3-4 (if primarily calcitic). Most collectible fossil corals are silicified.
• Specific Gravity: 2.58-2.64 (for silicified varieties); 2.71 (for calcitic varieties).
• Crystal System: Trigonal (for chalcedony/quartz) or Hexagonal (for calcite), but the material is cryptocrystalline or microcrystalline, preserving the original biogenic structure.
• Color: Wide range: white, gray, brown, black, pink, red, yellow, blue, often mottled or banded.
• Luster: Vitreous to waxy.
• Transparency: Opaque to translucent.
• Fracture: Conchoidal to uneven.
• Cleavage: None (for silicified); perfect rhombohedral (for calcitic, but rarely observed due to microcrystalline nature).
• Composition: Primarily silicon dioxide (SiO2) if silicified (chalcedony/chert), or calcium carbonate (CaCO3) if recrystallized calcite. Often contains trace impurities of iron, manganese, and other elements.

Quick Check

• Color: Variable, often earthy tones, but can be vibrant if silicified with impurities.
• Luster: Dull to waxy on unpolished surfaces; vitreous when polished.
• Streak: White (if silicified) or colorless (if calcitic), but generally not a useful diagnostic for this material.

Physical Characteristics

• Crystal Habit: Pseudomorph after coral skeletons; cryptocrystalline or microcrystalline aggregates.
• Cleavage Type: None (silicified); perfect rhombohedral (calcitic, but not typically observed).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

Formation

Fossilized coral forms when the original calcium carbonate (aragonite or calcite) skeleton of a coral polyp colony is replaced by silica (chert/chalcedony) or recrystallized into stable calcite through diagenetic processes. This replacement preserves the intricate internal and external structures of the coral colony. The process typically occurs in marine sedimentary environments where silica-rich fluids are present, often associated with volcanic activity or dissolution of siliceous organisms.

Usage

Primarily used as an ornamental stone for carvings, cabochons, beads, and polished display pieces. The unique patterns make it popular in lapidary arts. Historically, some cultures used fossilized coral for tools or decorative items. Petoskey Stone, a specific fossilized Favosites, is the state stone of Michigan and is highly sought after by collectors and artisans.

Age Distribution

Predominantly Silurian to Permian for Tabulate corals; Mesozoic to Cenozoic for Scleractinian corals. Favosites are characteristic of the Silurian and Devonian periods.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (if silicified, e.g., chalcedony) or Carbonate (if calcitic, e.g., calcite)

Group

Quartz group (if silicified) or Calcite group (if calcitic)

Crystal System

Trigonal (for quartz/chalcedony) or Hexagonal (for calcite)

Chemical Formula

SiO2 (if silicified) or CaCO3 (if calcitic)

Composition

Silicon dioxide (silicified) or Calcium carbonate (calcitic), with various mineral impurities.

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