Quartz, Chert, and Flint

Quartz commonly shows up in sedimentary rocks as rounded pebbles or as angular to rounded sand grains. Quartz grains freshly released from a weathered rock can be quite angular, but river transport followed by repeated pounding by ocean waves or eolian transport can make the grains remarkably spherical.

Chert and flint are microcrystalline varieties of quartz. Their quartz crystals are so tiny that chert and flint fracture more like glass than quartz crystals. A skilled person can chip chert and flint pieces into arrow heads, spear points, and other tools. The only difference between chert and flint is color: flint is black or nearly black and chert tends to be white, gray, or pink and can be either plain, banded, or preserve fossil traces.

This close-up of a sandstone shows a lot of quartz grains held together with a light gray calcite cement. This view is about natural size. Click on the image to get a larger view, and follow the link below to get a super-close up: Zoom Shot of Quartz Grains
Chert is great for making arrowheads and scrapers because the size of the tiny crystals is so small that the material has no prefered way of breaking. Thus, blows aimed by skillful hands can control the way the chert fractures . These were made by native peoples living in northern Mexico.
This is a cross-section of a geode showing alternating bands of crystalline quartz (you can see the crystas in the white/clear areas) and microcrystalline quartz (reddish bands). Variations in the rate of crystallization caused these alternating layers. Rapid crysallization generally makes small crystals and slow crystallization generally allows time for larger crystals to grow.

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