Colorless when pure; hexagonal; ZnS; mol wt., 97.43; sp gr, 3.98 to 4.1; Mohs hardness, 3.5 to 4.0; luster, resinous; transformation temperature from beta zinc sulfide to alpha zinc sulfide, 1,020+ or -5 degrees C: sublimes at 1,180 degrees C or 1,185 degrees C; melting point, 1,850 degrees C (at 150 atm); insoluble in water and in acetic acid; and very soluble in other acids. Occurs as the brownish-black mineral wurtzite, which is unstable compared with its stable dimorph, the mineral sphalerite (beta zinc sulfide), to which it inverts during alteration and from which it is formed by heating sphalerite to the transformation temperature. Can be crystallized from acid solutions above 250 degrees C.