So chert is also sometimes said to be a rock of chemogenic origin. It may move as a silica-rich liquid and form nodules in rocks by replacing the original (usually carbonate) material. Although the material it is made of ultimately came from siliceous tests of marine species, the rock itself is often not deposited in situ. Their siliceous tests are not made of quartz initially, but after burial, compaction, and diagenesis, opaline siliceous sediments transform to quartz. Sometimes microscopic fossilized remains of these sea creatures may be preserved in these rocks.
The latter is known as a banded iron formation (BIF) which is the principal iron ore upon which our society relies.Ĭhert is in most cases a biogenic rock, it is made of siliceous tests of diatoms, radiolarians, siliceous sponge spicules, etc. This rock is often bedded – rhythmically interlayered with chalk, shale or in some cases hematite. It occurs usually as nodules in carbonate rocks, especially well-known are chalks with chert (flint) nodules in Western Europe. White coloration is usually given by carbonate impurities organic matter or clay gives black color red, yellow, and brown tones are due to hematite, green variety may contain chlorite or smectite from diagenetically altered volcanic tuffs. Most common shades are gray, white, blue, green, yellow, black, and red. It may have many colors, depending on the nature of impurities. It is usually either dull or semivitreous. The boulder in the foreground is freshly broken to demonstrate typical sharp edges of chert which were eagerly exploited by our stone age ancestors. Gray rocks in the background are also flint boulders, but they lack fresh look because of mostly wave-agitated rounding. It is a common rock type which occurs mostly in carbonate rocks either in nodular form or in layers (bedded chert).įlint on the coast at Stevns Klint in Denmark. Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals.
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