Chert is a dense, hard siliceous-rock with low porosity made up of mainly of microcrystalline quartz and varying amounts of impurities. Cherts may form by compaction and recrystallization of silica-rich biogenic sediments made of opaline tests of single-cell organisms (diatoms, radiolaria) or remains of silicious sponges, both in marine and in lake environments. During diagenesis, the silica in the sediments undergoes a transformation from opal-A through opal-CT to microcrystalline quartz in the mature chert. Accordingly, these cherts may contain some opal-CT. Silica mobilized from volcaniclastic sediments, hydrothermal solutions and clay minerals may contribute to the silicification. [mindat.org]
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