Formula: Cu_2-xAl_x(H_2-xSi₂O₅)(OH)₄ · nH₂O, x < 1  Colour:  Green, bluish green, blue, blackish blue to black, or brown and rarely yellow  Lustre: Vitreous, Waxy, Earthy Hardness: 2½ – 3½  Specific Gravity:  1.93 – 2.4 Crystal System: Orthorhombic