Stone Tool Analysis: Hertzian cone
A Hertzian cone is a defining characteristic of conchoidally-initiated flakes. In Hertzian cone formation, the area in direct contact with the indentor—usually less than 2-3 mm in diameter—is depressed until the stone fails and a crack starts. The crack follows the boundary of the molecules compressed under the indentor. This boundary is cone-shaped with the sides flaring outward at ca. 136º to the direction of force.
If the blow is struck near the edge of the core, the radiating cone-shaped force encounters the stone’s outside or ‘free’ face; this causes the crack to change shape and run down the core’s face. The actual Hertzian cone is normally very short because the reconfiguration of the crack to the core face occurs so quickly. The cone is sometimes visible without magnification, and the ring crack and umbo at the top of the cone are often seen in fine-grained stones like chert, flint, and obsidian.