Quiz: Remember the definition of mal de mer?
"flint-like quartz," 1670s, of unknown origin. Apparently "a local term, which has been taken into geological use" [OED].
A hard, brittle sedimentary rock consisting of microcrystalline quartz. It is often reddish-brown to green but can also occur in a variety of other colors, especially white, pink, brown, or black. Chert often contains impurities such as calcium, iron-oxide, or the remains of silica-rich organisms. It usually occurs as nodules in limestone and dolomite and has curved fractures.