Favosites (Honeycomb Coral Fossil)
Favosites sp. (Genus of extinct Tabulate Coral) · Fossil / Mineral Specimen
Crystal System
Trigonal/Hexagonal (as Calcite); the biological habit is 'Favositoid' (honeycomb-like prismatic tubes)
Mineral Group
Carbonates (Calcite after biogenic structure)
Chemical Formula
CaCO3 (principally Calcite or Aragonite replacement)
Formation Process
Sedimentary/Perpetuation: Organic coral structure replaced by minerals (usually calcite or silica) from mineral-rich groundwater over millions of years.
About This Crystal
The specimen shows white, honeycomb-like geometric clusters on a dark gray matrix. These represent the cross-sections of polygonal corallites (individual tubes) where the coral polyps once lived. The luster is earthy to dull on the matrix, while the fossilized structures may appear slightly vitreous.
Physical Characteristics
Color: White to light tan (fossil) on dark gray/brown (matrix). Streak: White. Cleavage: Perfect rhombohedral in calcite areas. Fracture: Splintery to uneven. Specific Gravity: 2.7. Non-magnetic. Often reacts with HCl.
Optical Properties
Opaque to translucent in thin sections. If silicified, may show waxy luster; if calcified, exhibits double refraction in clear crystals, though usually massive here.
Hardness & Durability
Origin Region
Globally distributed; prominent in the Michigan Basin (USA), Morocco, and Europe. Found in Paleozoic sedimentary limestone beds.
Hardness & Durability
Mohs 3 (Calcite) to 7 (if Silicified). Calcite versions are brittle and easily scratched by steel; silica-replaced versions are quite durable.
Care & Maintenance
Clean with warm soapy water and a soft brush. Avoid acidic cleaners (vinegar, citric acid) as they will dissolve the carbonate fossil. Store away from harder minerals to prevent scratching.
Rarity & Value
Common. Values are generally low for small matrix pieces ($5-$20), though large, well-preserved polished colonies can command higher prices. Common in Devonian era strata.
Special Characteristics
Displays distinct 'Honeycomb' pattern created by the walls of the corallites known as 'mural pores'. It is a colonial organism structure.
Lore & History
Historically used as biological markers for the Silurian and Devonian periods (419-358 million years ago). Symbolically, fossils are often associated with longevity, ancient wisdom, and the passage of time.