A Guide to Some Of The Knappable Rocks Around The World


There are many different types of stone that can be used to make lithic tools, depending on where you are in the world. 'Lith' meaning stone (e.g. Megalith, Monolith, Microlith). A lithic is simply a stone tool, instead of a stone monument, that has been made from raw stone material; this covers all periods from all places over the globe. The term 'Lith' is also used within the stone tool categories, such as 'microlith' which was a type of tool in the British Mesolithic (a category of tools made from  very small flakes struck off a core, then retouched to make sharp cutting tools and points).


In Britain, flint and bio-sedimentary materials are predominantly found, whereas North America igneous material is more common. Different materials required different techniques to work them into effective tools. Some materials produce a sharp cutting edge naturally when flaked e.g. Obsidian and flint. Other materials have to be physically changed to make them useful e.g. some jaspers have to be heat treated to make them workable, as shown below...


Agate Arrowhead Replica's

AGATE - A common rock formation often found in metamorphic and igneous rocks; it is composed of silica, chalcedony and quartz primarily. Though generally used for ground and polished artwork and sculpture, agate fractures conchoidally making it suitable for knapping, though it is often best heat treated beforehand. Agate was commonly used in the USA to make arrowheads.




Waverley Wood: Andesite Handaxe

ANDESITE - An extrusive igneous volcanic rock of intermediate composition (between basalt and rhyolite). Its lower silica dioxide content (over other knappable rocks) of between 57-63% can make it hard to work. In the UK, andesite was used to make handaxes in Warwickshire (Waverley Wood Pit being the most well-known site) around 500,000 years ago.


New England Argillite Arrowheads

ARGILLITE - Argillite is a fine-grained sedimentary rock made of hardened clay particles. “Argillaceous” rocks are muds and fine silts that have been lithified into stone. Though quite fissile (splits into layers easily), the stone was used in North America and New Zealand to make axes, adzes and arrowheads.




BASALT - An extrusive igneous rock produced when magnesium and iron rich lava solidifies after being expelled onto the earth’s surface. More than 90% of the earth is made of basalt. Its low silica content of 45-53% makes it hard work to knap though it was used extensively for handaxes in central Africa.


CHALCEDONY - A cryptocrystalline (a crystal structure only visible under magnification) form of silica. Agate and Onyx are both types of chalcedony and all have been used for stone tools and ornamentation. Its use for stone tools is generally restricted to the Americas, though it occurs on most continents.


CHERT - A broad term used for any type of rock that is mainly cryptocrystalline quartz. It is sedimentary, often occurring in chalk, limestone, greensand and dolomite formations as a replacement mineral. Flint is often considered a chert along with jaspers and radiolarites (though these are primary minerals rather than replacement). Cherts occur almost all over the world and has been used for stone tools where humans have had access to it.


Montanna, USA Dacite Arrowhead ©Geology.com

DACITE - An igneous rock (felsic extrusive) of intermediate composition between andesite and rhyolite. It often forms lava flows or intrusions at the centre of old volcanoes. First described in the Roman province of Dacia (Romania) with a silica content of 63%-69%. Dacite occurs in many European countries including Scotland, Germany, France and Spain; though it is in the USA where it was used to make stone tools.



FLINT - A cryptocrystalline rock that falls under the chert group. Like sedimentary cherts it is a replacement mineral through diagenesis (the change of sediment to rock through pressure and chemical change). Flint (hardness 7 on the Mohs scale) typically has a glassy lustre and can be flaked with limited effort. Like other sedimentary cherts, it typically forms in nodules between layers of chalk or limestone.



Ignimbrite Handaxe

IGNIMBRITE - A pumice-dominated pyroclastic flow deposit, also known as a hardened tuff. Pyroclastic flow material is emitted from an explosive eruption before settling in thick layers. The layers weld together due to the extreme heat (over 500°C) of pyroclastic material. It's fine-grained, glassy matrix makes it knappable, but without the glassy lustre of obsidian. It was used for making acheulean handaxes in central and northern Africa.



Jasper Stone Tool Replica's

JASPER - A cryptocrystalline quartz that can be aggregated with chalcedony. It can range in colour from yellow, brown and green though it is most common red. It can fall into any of the three main rock groups of igneous, metamorphic or sedimentary meaning it can occur with a broad variety of different properties. One constant is its high silica content which gives a conchoidal fracture, this makes it workable by flaking. It has been used to make stone tools on most continents.


Arkansas Novaculite Arrowheads

NOVACULITE - A cryptocrystalline rock that occurs in the US, Japan and Middle East. It is primarily made of silica from hard shells of organisms like diatoms and wind-blown quartz particles. Following diagenesis (at which point it is chert), uplift and mild metamorphism turns the chert to Novaculite. It can be worked exactly like flint, though some types require heat treatment.



Obsidian Arrowheads ©National Park & Preserve Alaska

OBSIDIAN - Natural volcanic glass (extrusive igneous) produced from felsic lava. High silica content (over 70%) of fast moving lava and rapid cooling (preventing crystal growth) results in obsidian. Over time, obsidian becomes fine-grained silica crystals meaning now samples have been found that predate the cretaceous period (the process is accelerated by water). Obsidian was widely used for tools an ornamentation in Europe, the Americas, Asia, Australasia and Africa. More recently it has been used for surgical scalpels as it can provide a cutting edge of 1 molecule thickness.


Onyx

ONYX - Similar to agate (both are forms of chalcedony), onyx is parallel banded while agate has curved bands. The colours of the bands in onyx are typically black or white, but can range to any colour. It occurs in Europe, Asia, Africa and the Americas and was typically used for polished ornamentation. It was not commonly used for making stone tools, though modern knappers (mainly in the US) occasionally work it.


Opal Arrowhead - ©Peter A. Bostrom

OPAL - Opal is unusual as it is hydrated silica (SiO2·nH2O). Its water content can range from 3 - 21% (by weight), typically 6 - 10%. It is often deposited within cracks and fissures, it can occur within almost any kind of rock. It was generally used for ornamentation in history, but is used by modern knappers.


Petrified Wood Tools ©Yale Peabody MoNH.

PETRIFIED WOOD - A name given to specific types of plant fossil. The “petrification” process begins when the organic material that makes up the tree cell walls are replaced by mainly silica rich minerals such as opal, chalcedony and quartz. The process can only occur when the tree is buried in silica-rich soil saturated with water. The fossil material has been used around the world for stone tools, but particularly in Africa and the Americas.



PORCELANITE (SILICEOUS SHALE) - A highly unusual rock, sometimes spelt with two “L”s. Porcelanite is a tough, contact metamorphosed hornfels from a basaltic intrusive or extrusive sequence. It can be found in Northern Ireland at Tievebulliagh and Rathlin Island, at both locations it was quarried for Neolithic axe heads which were ground smooth in the nearby area.




Quartz Tools ©Sharma Centre, Heritage Education, India

QUARTZ - The most abundant mineral found on the earth's surface, quartz is essentially silica with two parts oxygen. Quartz is found within and makes up many of the knappable rocks and modern materials (such as glass). There are many forms of quartz and it is highly knappable in its pure, crystalline form. Quartz has been used for stone tools almost everywhere people have made tools and quartz has occurred (so most countries).



Quartzite Handaxe ©Hull & East Riding Museum

QUARTZITE - Originally quartz sandstone (sedimentary) before being metamorphosed by heat and pressure in quartzite. In its pure form it is white/grey, but can be pink to red depending on the iron content. Quartzite pebbles make excellent hard hammers for flintknapping due to its hardness. It was used in prehistory for handaxes in Africa, Europe and Asia. However it is very hard work to knap (unless it is a high-silica variant such as Orthoquartzite.


Rhyolite Arrowheads ©Geology.com

RHYOLITE - Rhyolite is an igneous rock, of felsic composition (69% silica oxide). It can form with a glassy texture, making it more suitable for knapping, though generally is porphyritic (varying crystal size). The mineral assemblage of rhyolite is usually quartz, sanidine and plagioclase. Rhyolite was quarried from around 11,500 years ago in Pennsylvania (US) for spear and arrowheads.




SILICIFIED CORAL - Coral which has undergone replacement with agate, chalcedony or quartz (see above). Varies widely in silica content and suitability for knapping and generally requires heat treatment before flaking.


Silicified Mudstone Axehead/adze © E. Cornelissen, RMCA.

SILICIFIED MUDSTONE - Can occur as large sheets, irregular nodules, tube-like marine animal burrows and internal molds of molluscs. Silica-rich forms of silicified mudstone are fine-grained, have a conchoidal fracture, and retain a sharp cutting edge which makes it suitable for knapping. Silica poorer forms are grainy with inclusions, making it less predictable to flake.


SILICIFIED SHALE - Silicified shale is similar in appearance to Porcellanite (see above), though tends to flake with more splinters owing to the nature of the original shale (very fissile). This type of shale is rather rare as an in between of sedimentary shale and metamorphosed shale known as “slate”. Most shales are compressed and lithified clay, though silicified shale is created from silica-rich clay.


SILICIFIED SILTSTONE - Similar to silicified shale, though far less common. Siltstone itself is best described as an extremely fine grained sandstone. The grains have been transported by wind or water.





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