How to identify Fossilized Coral in Chert

To correctly identify Fossilized Coral in Chert (Chert with coral fossil inclusions), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable, depending on impurities in the chert and the preservation of the coral. Chert matrix can be white, gray, black, brown, red, yellow, or blue. Fossilized coral structures often appear in contrasting colors or shades, sometimes white, cream, or light brown against a darker matrix, or vice-versa.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous on freshly broken surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Microcrystalline to cryptocrystalline, very fine-grained. The coral structures may exhibit a slightly coarser texture or distinct patterns (e.g., septa, corallites) that are visible to the naked eye or under magnification.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Massive, nodular, or bedded. The individual quartz crystals are microscopic and anhedral. Coral fossils retain their original biological structures.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its microcrystalline nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in marine sedimentary environments, often associated with limestone or chalk formations, where silica-rich fluids have replaced calcium carbonate. Can occur in nodules, beds, or as replacement of existing carbonate rocks.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to the chert matrix).

Specific Gravity

2.58-2.64 g/cm3.

Crystal System

Trigonal (for the constituent quartz crystals, though the rock itself is massive).

Color

Highly variable, often with contrasting colors between the chert matrix and the fossilized coral.

Luster

Waxy to dull, sometimes vitreous.

Transparency

Opaque to translucent.

Fracture

Conchoidal.

Cleavage

None.

Composition

Primarily Silicon Dioxide (SiO2) in the form of microcrystalline quartz (chalcedony), with trace impurities (e.g., iron oxides, manganese oxides, organic matter) that impart color. The coral structures are also silicified SiO2.

Physical characteristics

• Crystal Habit: Massive, cryptocrystalline aggregates. Coral structures retain their original biological morphology.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal, producing sharp edges.
• Tenacity: Brittle.
• Luster Type: Waxy, dull, sometimes vitreous.

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