Knappable Lithics
This is not intended to be a comprehensive list of flakable
stone, but it does cover many of the broad categories of stone which can
be knapped. It was compiled by Dean Pritchard ([email protected])
and Tim Rast.
- Agate
- A semiprecious variety of chalcedony and identical with
quartz in composition and physical properties. Agate is available in many
colors do 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.
- 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.
- Argillite
- See Silicified Siltstone.
- 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.
- Chalcedony
- Chalcedony is a fine-grained, microcrystalline silica
rock. It has a waxy luster and a splintery fracture. Agate, carnelian,
bloodstone, sardonyx, and others are semiprecious varieties of chalcedony.
Knapping characteristics can be improved with heat-treating.
- 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. Chert is also the name for the silica in petrified
wood and many fossils. Impurities in the chert result in a wide variety
of colors and texture. Because the differences between flint and chert
are so subtle, many archaeologists refer to chert, chalcedony, and flint
as 'Crypto- Crystalline Silicates', or CCS. Most chert will respond to
heat-treatment.
- 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. see also
chert.
- 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.
- 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.
- 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.
- 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.
Check out this cool web site dedicated to Obsidian.
- Onyx
- A variety of agate with numerous alternating, parallel
bands of cloudy white, dark brown or black colored impurities.
- 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.
- Petrified Wood
- Petrified wood which has had the original organics replaced
by minerals and the pore space filled with silica can be a very workable
material. Many pieces, however, fracture along the wood grain. Solid, agatized
or opalized wood works best in the lithic reduction process.
- Porcellainite
- Forms below the surface at the interface between clay
and coal beds. The clay is baked into a natural ceramic, hence the term
Pocellainite. Can be difficult to pressure flake.
- Quartz
- Quartz is basically pure silica. Large quartz crystals
can be knapped. Not to be confused with quartzite, which is composed of
many tiny quartz crystals which have been cemented together. Many of the
other flakable stone types, such as chert, chal cedony, japser, and petrified
wood are quartz (silica dioxide) bonded with different amounts of water.
- Quartzite
- Metamorphosed Sandstone. 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.
- Rhyolite
- Rhylotite 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. Gas-rich rhyolite erupts violently
to form welded tuffs, or ignimbrites, and may spread out over great distances.
Rhyolite is very difficult to knapp.
- 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.
- 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.
- 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.
- 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. Siltsones, however, generally lack the fissility
of shale. Heat-treating is generally not recommended.
If you have any definitions or comments on any of these material
types that you would like to see added, please Mail me.
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