What is a jasper in geology ?

Jasper : definition

Jasper is essentially a siliceous sedimentary rock, containing no detrital fragments, resulting from the diagenesis of a radiolarian mud. Silica makes up more than 90% of the rock, in the form of chalcedonous radiolarians, quartz or chalcedonous cement, rarely opaline. These very fine-grained rocks, with scaly or conchoidal fracture, are gray or variously colored, the most common varieties being phtanites, lydians and radiolarites.

Striped jaspers, or simply aesthetic, are used in jewelry and ornamentation.

However, some authors consider jasper to be a microcrystalline variety of quartz (chalcedony), with a mat luster, most frequently colored red by thin layers of hematite.