Chert and porcellanite are field terms used by shipboard scientists to differentiate fine-grained siliceous sedimentary rocks based on their textural and physical properties (Hesse, 1990). Porcellanite is the term used to describe a porous siliceous sedimentary rock with a dull or matte luster similar to unglazed porcelain, whereas chert is the term used to describe a dense, vitreous, hard, and brittle rock (Bramlette, 1946). Pisciotto (1980) defines chert similarly as a hard, conchoidally fracturing, cryptocrystalline, varicolored sedimentary rock with semivitreous, vitreous, or waxy luster, consisting dominantly of silica. He describes porcellanite as a siliceous sedimentary rock with a dull or matte luster resembling that of unglazed porcelain that is less hard, dense, and vitreous than chert and commonly has lower silica content.
Siliceous rocks have also been categorized by mineralogical composition. Isaacs (1981a) classifies siliceous rocks mainly on the relative abundance of silica and detrital minerals; silica ranges from 20% to 90%, and the ratio of silica to detrital minerals ranges from 0.3 to 9.0. Where silica is diagenetic, physical properties are closely related to the silica/detrital ratio. This ratio is >3.0 (~75% silica) in cherts and cherty porcellanites, 1.25–5.0 (~22%–55% silica) in porcellanites, and <1.0 (~50% silica) in siliceous shales and siliceous mudstones. Opal A is transformed to quartz at increasing temperatures, decreasing the silica to detrital mineral ratio, and opal-CT subsequently transforms to quartz at increasingly lower temperatures (Isaacs, 1981b). Jones and Murchey (1986) also use detrital clay mineral content to differentiate between chert and porcellanite. The higher (25%–50%) clay mineral content corresponds to porcellanite, whereas the lower clay content (0%–25%) corresponds to chert.
Cherts of deepwater origin can also be bedded or nodular, with nodular structures ascribed solely to diagenetic processes and bedded units ascribed to variations in surface water productivity or terrigeneous sediment input, overprinted by diagenesis (Hesse, 1990).