Color is dark green to blue green or less commonly brown, gray and black. Luster is vitreous to pearly to dull. Crystals are generally translucent to opaque. The typical termination appears to be two faces of a slightly slanted dome but is actually two of the four faces of a prism. The termination faces are not only slanted toward each other but the two faces are slanted with respect to the long axis of the crystal as well.
Some terminations are rather complex and can make the crystal appear pseudo-orthorhombic.
Due to dissolution reactions after initial crystallization, the crystal faces can appear pitted and the terminations can be rounded. Cleavage is imperfect in two directions at nearly 60 and 120 degrees. Fracture is uneven. Hardness is 6. Streak is white.