"Rock of the Month # 79, posted for January 2008" ---

This flattened slab of flint, roughly 20 cm in diameter, is a good example of the form of chert (fine-grained, colloidal silica) found in the Cretaceous Chalk of western Europe. Flint is common in the fields of the East Anglian region of southeast England (Greensmith et al., 1973; Lucy, 1999). Digital photo taken January 2008.

The shell-like (conchoidal) fracture is a characteristic feature of this rock type, explaining its suitability for knapping (shaping) into tools and weapons. It is common in cherts around the world (e.g., Wray, 1948). See also Flint as a building stone (Rock of the Month 34). In addition to flint, conchoidal fracture is commonly reported in shiny to glassy hydrocarbons, including cannel coal, shungite, jet and amber. It is reported in some volcanic rocks (some andesites, also obsidian), in siliceous weathering horizons (silcretes), and in some discrete mineral species, such as garnets and corundum.

References

Greensmith,JT, Blezard,RG, Bristow,CR, Markham,R and Tucker,EV (1973) The Estuarine Region of Suffolk and Essex. Geologists' Association Guide 12, 41pp.

Lucy,G (1999) Essex Rock, a Look Beneath the Essex Landscape. Essex Rock and Mineral Society, 128pp.

Wray,CF (1948) Varieties and sources of flint found in New York state. Pennsylvania Archaeologist 18 nos.1-2, 25-45.