Generally speaking, tools that require a sharp edge are made using cryptocrystalline materials that fracture in an easily controlled conchoidal manner. Cryptocrystalline tool stones include flint and chert, which are fine-grained sedimentary materials; rhyolite and felsite, which are igneous flowstones; and obsidian, a form of natural glass created by igneous processes. These materials fracture in a predictable fashion, and are easily resharpened. For more information on this subject, see lithic reduction.
Large-grained materials such as basalt, granite and sandstone may also be used as tool stones, but for a very different purpose: they are ideal for ground stone artifacts. Whereas cryptocrystalline materials are most useful for killing and processing animals, large-grained materials are usually used for processing plant matter. Their rough faces often make excellent surfaces for grinding plant seeds. With much effort, some large-grained stones may be ground down into awls, adzes, and axes.
- Andrefsky Jr., William (2005). Lithics: Macroscopic Approaches to Analysis (Second ed.). Cambridge University Press. ISBN 0-521-61500-3.
- Daniel S. Amick (1999). Folsom lithic technology: explorations in structure and variation. International Monographs in Prehistory. ISBN 978-1-879621-27-5. Retrieved 2010-10-03.