Chips coming off a flint can have razor sharp edges, making it suitable as a cutting tool (it's a good idea to wear safety goggles if you work on flint).įlint is easy to spot in a gravel pit: often it is covered by a thin white layer, and - in contrast to the other pebbles - it has an irregular shape. This depends a little bit on the amount of impurities, "purer" flint behaves more like glass. It's easier to control the direction of the splitting, and the edges are more straight. Flint has a conchoidal fracture like rock crystal or glass, but its fracture surfaces are not as uneven and curved. The most interesting physical property of flint is the way it splits. Although it has a powder-like consistence and can sometimes be partially rubbed off, it is not chalk, but quartz (you cannot dissolve it in acids). Flint will crack in fire because of the water in it, sometimes so badly that small flint chips fly around.įlint concretions from sedimentary rocks forms irregular nodules that are often surrounded by a thin white layer, sometimes called cortex. After a couple of years they have mostly dried out and get more brittle. ![]() ![]() The walls of the cavities are often made of gray, white or blue, translucent chalcedony.įlint freshly removed from chalk contains a few percent of water. Small cavities lined with small quartz crystals (usually less than a millimeter in size) are not uncommon in large flint nodules. The size of the grains in flint is between 0.5 to 20 micrometers ( Knauth, 1994). Jasper is almost opaque and typically its colors are more intense, while flint is often a bit translucent. Like jasper, it has a very irregular, grainy structure, whereas agates - also a cryptocrystalline quartz variant - consist of regularly intergrown tiny quartz crystals that give them a "fibrous" structure. It contains considerable amounts of other silica modifications, mostly moganite ( Heaney and Post, 1992), perhaps opal. It is a textural variety of quartz that shares some properties with jasper. Some people would say that flint and chert are technically spoken not minerals, but rocks. On freshly broken surfaces the luster is dull, at best waxy, but because it is very hard, flint takes a good polish and assumes a glassy luster, just like agate.įlint is not a chemically very pure quartz variety, the large amounts of impurities and its fine-grained structure can make it dull and almost opaque. It is slightly translucent to almost opaque, sometimes only thin chips are translucent at the edges. The color can be caused by inclusions of organic compounds (black), metal sulfides (black), and various metal oxides and hydroxides (yellow, orange, brown, reddish, etc.). Flint may show color banding, but this is not a concentric banding as seen in agate. If a large body of rock is entirely made of dense, dull cryptocrystalline quartz, it is generally called chert, regardless of its color. Brighter or more colorful variants are sometimes called chert by some people. It is possible that once I've been working through the literature more deeply I change my mind and lump chert, flint (and possibly jasper) together.įlint does not have a specific color, but is often dark gray with shades of brown, red, or yellow, and sometimes white. This is done not so much out of respect for the "tradition", but because I'm not sure how "special" flint is in its properties, and how broadly the term chert is defined with respect to structural and physical properties. However, I will use the term "flint" for the nodular forms and the term "chert" for the rock-forming type. In that sense, Knauth did the right thing to call flint "nodular chert", because he considers them equal in their basic physical properties. Otherwise the nomenclature will turn quickly into a inconsistent mess, once old assumptions about the formation must be revised. ![]() Knauth, 1994) refer to flint as "nodular chert".Īs a general rule, one should primarily categorize rocks according to what they are, and not to how they formed. This chapter is called "Flint and Chert", but after looking at the matter again and again, I'm getting less and less convinced that chert and flint should be treated together in a chapter, although some authorities on that matter (e.g. There's apparently no clearly defined line that separates flint from chert and that people agree on. Both terms are more often used very broadly. Others use the term "flint" for nodules and "chert" for large bodies of rocks. Some authors use "chert" as the more general term, with flint being a dark variant. Firestone, Hornstone, and Silex are other names for flint and chert. Flint and chert are dense, cryptocrystalline varieties of quartz, slightly translucent to almost opaque.
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