Lydite

A black, carbonaceous, radiolarian chert: a recrystallised radiolarite, with low porosity. Also used, confusingly, as a synonym of basanite, a basaltic rock. Both were once commonly used as touchstones for testing gold alloys.

A sedimentary silica-rich rock belonging to the group of organogenic rocks, formed mostly from radiolaria, it is black or dark gray, cryptocrystalline or very finely crystalline. It consists mainly of chalcedony and/or microcrystalline quartz with minor organic matter and often trace pyrite. The fluctuating content of clay minerals and pyroclastic components commonly causes strong layering. During diagenesis, as a result of chalcedony recrystallisation and remobilisation, quartz may infill the porosity and form veins, while the organic components are highly carbonised.