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Agate
A semiprecious variety of chalcedony and identical with quartz in composition and physical properties. Agate is available in many colors due to iron hydroxide and other impurities. Although agate has a natural conchoidal fracture, it is best to heat-treat it for the lithic reduction process.
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Andesite
These rocks are abundant in island arcs and along active continental margins. They are commonly lighter-colored and less heavy than basaltic rocks and contain no quartz. Andesite lavas, named for the Andes Mountains, are intermediate in chemical composition between the basaltic-gabbroic and rhyolitic-granitic rocks.
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Argillite
See Silicified Siltstone.
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Basalt
Basalt is a fine-grained, silica-poor, igneous rock and is by far the most abundant volcanic rock. It ranges from black to dark gray in color. Most basalts occur in lava flows and sheets. Basalt is very difficult to knapp.
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Chalcedony
Chalcedony is a fine-grained, microcrystalline silica rock with a waxy, smooth luster. It will range from transparent to translucent with the most common colors being white to gray. Agate, carnelian, bloodstone, sardonyx, and others are semiprecious varieties of chalcedony. Knapping characteristics can be generally improved with heat-treating.
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Chert
A silica-rich crypto-crystalline rock. Cherts will form from a variety of sedimentary rock processes and are commonly found as nodules in limestone & dolomite deposits. Chert is generally considered an impure form of flint. Impurities in the chert result in a wide variety of colors and textures. Because the differences between flint and chert are so subtle, many archaeologists refer to chert and flint as “Crypto-Crystalline Silicates” or CCS. Most chert will respond to heat-treatment.
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Dacite
Dacite is an extrusive igneous rock that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium. It is similar to Andesite, but containing less sodic plagioclase and more quartz.
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Flint
Flint is a sedimentary rock and consists predominantly of silica, with occasional impurities such as calcite, iron oxides, clay minerals, and the organic remains of marine organisms also made of silica. A characteristic of flint is its natural conchoidal fracture which makes it a favorite among flintknappers. Flint ranges from opaque to semi-translucent, and colors may range from white to one of various shades of gray, brown, or black. Flint occurs principally as nodules or concretions in limestone and chalk beds. Better grades of flint require no heat-treatment.
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Ignimbrite
An igneous rock, similar to obsidian, except it is opaque and somewhat grainy. It is generally considered an impure form of obsidian. These rocks are made of ash and other debris shot out during explosive rhyolitic volcanic eruptions. In major rhyolitic explosive eruptions, glowing avalanches of ash blasted from a volcano are so hot that they weld together upon settling, forming ignimbrite or welded tuff. Lower grades of ignimbrite will be difficult to knapp.
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Jasper
Essentially metamorphosed chert. Jasper is fine-grained and homogeneous in texture. It may also have a waxy appearance. Jasper is an opaque to slightly translucent stone with a tendency towards bright colors, reds, browns, olives, and yellows. Most jasper will be improved by heat-treating.
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Novaculite
Novaculite is a metamorphic rock derived from a sedimentary rock known as bedded chert. It is a very dense and hard, even-textured, light-colored cryptocrystalline, siliceous sedimentary rock, similar to chert, but characterized by microcrystalline quartz. Novaculite is used as a “whetstone” for sharpening cutting instruments. It almost always requires heat-treating in order to make it knappable.
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Obsidian
Obsidian, a natural volcanic glass, usually of rhyolitic composition, forms by rapid cooling of a silica-rich viscous lava. Most obsidians are more than 70 percent silica. Obsidian occurs as thick, short flows or domes over volcanic vents. It is usually black but occasionally found in other colors. Obsidian displays a well-developed conchoidal fracture, which makes it a favorite among flintknappers. Do not heat-treat obsidian.
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Onyx
A variety of quartz with numerous alternating, parallel bands of cloudy white, dark brown or black colored impurities. Onyx generally has a vitreous luster and breaks with an uneven fracture.
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Opal
Opal is an opaque hydrated silica stone often found with brilliant color flashes. Opals have a relatively high water content and are porous; they will lose their color over time and may crack if they dry out. They may also absorb body oils or other liquids. Opal requires careful attention during the knapping process; it breaks easily.
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Petrified Wood
Petrified wood which has had the original organics replaced with silica can be a very workable material. Many pieces however, will fracture along the original wood grain. Homogeneous, agatized or opalized wood works best in the lithic reduction process.
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Porcelainite (Siliceous Shale)
Forms below the surface at the interface between clay and coal beds. The clay is baked into a natural ceramic, hence the term Porcelainite. It is very difficult to pressure flake.
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Quartz
Quartz is basically pure silica. Large quartz crystals can be knapped. Many of the other flakeable stone types, such as chert, chalcedony, japser, and petrified wood are quartz (silica dioxide) bonded with different amounts of water.
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Quartzite
Metamorphosed Sandstone and extremely variable. The quality depends on the coarseness of the original sandstone (the finer the better) and how tight the bond between the sand grains becomes (the stronger the better). It can be chipped, but is difficult to work.
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Rhyolite
Rhyolite is an extrusive igneous rock with the same composition as granite. It is a light-colored, fine-grained volcanic rock with a very high (more than 70%) silica content. It often contains phenocrysts of quartz and feldspar in a glassy matrix. Rhyolite is always translucent to some degree. Gas-rich rhyolite erupts violently to form welded tuffs, or ignimbrites, Rhyolite is often difficult to knapp.
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Silicified Coral
Fossilized coral - sometimes called “agatized coral”, is found in two forms, as geodes, which represent partial replacement of coral, and as solid pieces which represent total replacement. Coral polyps may be visible as star or snowflake shaped patterns in the stone. It is a very tough and grainy rock. Colors will range from shades of white, tan and gray. Most silicified coral must be heat-treated to be knappable.
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Silicified Mudstone
A fine-grained detrital sedimentary rock made up of lithified silica-rich clay and silt-sized particles. Some mudstones will display mud cracks; a result of lake bed sediment exposed above water. Better quality silicified mudstone will typically range in various shades of brown. Heat-treating is generally not recommended.
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Silicified Shale
A fine-grained sedimentary rock made from compacted and lithified silica-rich clay. The clay consists of hydrous silicate minerals consisting mainly of silica, aluminum, and water. It is formed in quiet water environments, such as lake bottoms, where the finest particles settle to the bottom. Shales may also contain fossils of the animals or plants from these environments. One notable property of shale is its fissility, the property of splitting easily along bedding planes into thin layers. Heat-treating is generally not recommended because of the entrapped water content.
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Silicified Siltstone
Silicified Siltstone is closely related to the silicified shale described earlier. It is formed from slightly larger particles of silt under similar conditions; completely opaque. Siltsones, however, generally lack the fissility of shale. Heat-treating is generally not recommended.
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