Siliceous rocks

Siliceous rocks include all rocks rich either in silica (SiO2) or silicates (SiO4). It is a big group of rock, and is a very common group of rock.

All igneous rocks are siliceous rock. In sedimentary rock, most (not all) clastic rocks are siliceous. In contrast, most chemical sedimentary rocks are not siliceous. Since it is not always easy to tell a siliceous rock from a carbonate rock, simple tools like a scratching knife or a bottle of dilute acid would be very handy to tell them apart.

Common siliceous chemical rocks include varieties of silica, such as: flint, chert, jasper, agate, chalcedony, etc.

A chert, harder than knife, no reaction with HCl
A limestone, softer than knife, bubbles in acid