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Chert

Chert (microcrystalline quartz) (SiO2) includes chalcedony, agate, jasper, and flint. Chert and flint are so similar that there is no sharp distinction between them. Dark-colored nodules are called flint, and the light-colored variety is called chert. The variation may be due to the inclusion of variable amounts of organic matter. Other chert colors can include pink, brown, and purple. Chert has a conchoidal fracture, a hardness of around 7, a dull luster, and a colorless streak.

Arrowheads in Texas and elsewhere were commonly crafted from chert.

Chert (microcrystalline quartz) (SiO2) includes chalcedony, agate, jasper, and flint. Chert and flint are so similar that there is no sharp distinction between them. Dark-colored nodules are called flint, and the light-colored variety is called chert. The variation may be due to the inclusion of variable amounts of organic matter. Other chert colors can include pink, brown, and purple. Chert has a conchoidal fracture, a hardness of around 7, a dull luster, and a colorless streak. Arrowheads in Texas and elsewhere were commonly crafted from chert.

The irregular masses of chert that are common in Texas limestones may have been formed by silica-rich groundwater passing through the sediments before they were lithified (formed into rocks). These irregular masses, known as nodules, often have exotic shapes uninfluenced by the textures in the host limestone.

The chert sample in the Texas Rock Kit was collected in Bexar County and is Cretaceous in age.