40d:Chert

Chert

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Uses
Masonry Stone crafting Construction
Properties
Sedimentary layer Material value 1
Contains
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Wikipedia article

When a chert stone is struck against steel, sparks result.

In prehistoric times, chert was often used as a raw material for the construction of stone tools.

The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallisation and metamorphism has made it an ideal rock for preservation of early life forms.

There is much confusion concerning the exact meanings and differences among the terms "chert", "chalcedony" and "flint" (as well as their numerous varieties). In petrology the term "chert" is used to generally refer to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz. The term does not include quartzite. Chalcedony is a microfibrous (microcrystaline with a fibrous structure) variety of quartz. Strictly speaking, the term "flint" is reserved for varieties of chert which occur in chalk and marly limestone formations. The distinction between "flint" and "chert" is often one of quality - chert being lower quality than flint. Most true flint (that found in chalk formations) is indeed of better quality than "common chert" (from limestone formations). Chalcedony is sometimes considered separately from chert due to its fibrous structure. Since many cherts contain both microcrystaline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.

There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics. Some of the more common varieties are:

• Flint is a compact microcrystaline quartz. It is found in chalk or marly limestone formations and is formed by a replacement of calcium carbonate with silica. It is commonly found as nodules. This variety was often used in past times to make bladed tools.
• "Common chert" is a variety of chert which forms in limestone formations by replacement of calcium carbonate with silica. This is the most abundantly found variety of chert. It is generally considered to be less attractive for producing gem stones and bladed tools than flint.
• Jasper is a variety of chert formed as primary deposits, found in or in connection with magmatic formations which owes its red color to iron(III) inclusions. Jasper frequently also occurs in black, yellow or even green (depending on the type of iron it contains). Jasper is usually opaque to near opaque.
• Radiolarite is a variety of chert formed as primary deposits and containing radiolarian microfossils.
• Chalcedony is a microfibrous quartz.
• Agate is distinctly banded chalcedony with successive layers differing in colour or value.
• Onyx is a banded agate with layers in parallel lines, often black and white.
• Opal is a hydrated silicon dioxide. It is often of a Neogenic origin. In fact is not a mineral (it is a mineraloid) and it is generally not considered a variety of chert, although some varieties of opal (opal-C and opal-CT) are microcrystaline and contain much less water (sometime none). Often people without petrological training confuse opal with chert due to similar visible and physical characteristics.
• Magadi-type chert is a variety that forms from a sodium silicate precursor in highly alkaline lakes such as Lake Magadi in Kenya.

Other lesser used terms for chert (most of them archaic) include, firestone, silex, silica stone and flintstone.