Color is deep blue almost black to a lighter indigo blue. Luster is vitreous to pearly. Crystals are usually translucent but some exceptional specimens are transparent. The cubes are rarely over half an inch on each side. Individual twinned crystals are common and are often found loose without any host matrix.
Cleavage is perfect in one direction.
Fracture is uneven and brittle. Streak is light greenish-blue. Notches or interpenetrant angles can be seen in some specimens revealing their true twinned nature. Associated Minerals include other rare copper and lead chlorides such as pseudoboleite , cumengite , chloroxiphite , paralaurionite , matlockite , chlorargyrite , bideauxite , atacamite , mendipite and diaboleite . Also found with cerussite , smithsonite , leadhillite , linarite , phosgenite and chrysocolla .