Cuprite is the name given to a naturally occurring copper oxide mineral. It is often a bright red colour and very attractive. Normally cuprite occurs as cubes or octahedra (it is a member of the cubic or isometric crystal system).
Occasionally, it grows more rapidly in one direction and forms long capillary crystals, a habit known as 'chalcotrichite', but these crystals still have a square cross-section.
This image is of a specimen of chalcotrichite from Nasbinal, Lozere, in France.