Chert

Chert is typically non-toxic unless it's in powder form. Long-term exposure to finely ground powder may lead to silicosis. Therefore, when cutting or polishing chert, be sure to wear a dust mask to avoid inhaling its dust.

Chert is a form of quartz, characterized by its microcrystalline or cryptocrystalline structure. This unique structure imparts an exceptional level of toughness to the stone, making it highly resistant to chipping or cracking under everyday wearing conditions.

Chert is stable in everyday conditions due to its inherent resistance to light exposure and contact with common household detergents. It retains its appearance well even when exposed to moisture or dry conditions and is not easily affected by contamination.

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

The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallization and metamorphism has made it an ideal rock for preservation of early life forms. For example: The 3.2 Ga chert of the Fig Tree Formation in the Barberton Mountains between Swaziland and South Africa preserved non-colonial unicellular bacteria-like fossils. The Gunflint Chert of western Ontario (1.9 to 2.3 Ga) preserves not only bacteria and cyanobacteria but also organisms believed to be ammonia-consuming and some that resemble green algae and fungus-like organisms. The Apex Chert (3.4 Ga) of the Pilbara craton, Australia preserved eleven taxa of prokaryotes. The Bitter Springs Formation of the Amadeus Basin, Central Australia, preserves 850 Ma cyanobacteria and algae. The Rhynie chert (410 Ma) of Scotland has remains of a Devonian land flora and fauna with preservation so perfect that it allows cellular studies of the fossils.

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Chert was once a popular stone for creating blades and weapons because it is hard and has a very particular fracture pattern. However, in the modern day it is more often used as an aggregate material for construction, though it is not the most popular choice as it can crack easily.

Chert occurs in carbonate rocks as oval to irregular nodules in greensand, limestone, chalk, and dolomite formations as a replacement mineral, where it is formed as a result of some type of diagenesis. Where it occurs in chalk or marl, it is usually called flint. It also occurs in thin beds, when it is a primary deposit (such as with many jaspers and radiolarites). Thick beds of chert occur in deep marine deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas, Oklahoma, and similar occurrences in Texas and South Carolina in the United States. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides. Chert also occurs in diatomaceous deposits and is known as diatomaceous chert. Diatomaceous chert consists of beds and lenses of diatomite which were converted during diagenesis into dense, hard chert. Beds of marine diatomaceous chert comprising strata several hundred meters thick have been reported from sedimentary sequences such as the Miocene Monterey Formation of California and occur in rocks as old as the Cretaceous.

In petrology the term "chert" is used to refer generally to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz. The term does not include quartzite. Chalcedony is a microfibrous (microcrystalline 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. Among non-geologists, the distinction between "flint" and "chert" is often one of quality – chert being lower quality than flint. This usage of the terminology is prevalent in North America and is likely caused by early immigrants who brought the terms from England where most true flint (that found in chalk formations) was indeed of better quality than "common chert" (from limestone formations). Among petrologists e.g. Philip King, Robert B. Neuman, Jarvis B. Hadley, chalcedony is sometimes considered separately from chert due to its fibrous structure. Since many cherts contain both microcrystalline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.