Smithsonite is a distinctive zinc carbonate mineral (ZnCO3). It typically forms finely-crystalline botryoidal layers and coatings. It's characterized by having a nonmetallic, subvitreous luster, just about any color, a hardness of 4 to 4.5, a white streak, rhombohedral cleavage (this can only be seen if nice, decent-sized crystals are present), and is moderately heavy for its size. It bubbles slightly in acid.
Smithsonite forms in supergene environments from the breakdown of sphalerite. It also occurs in some hydrothermal veins.
Smithsonite (field of view 2.9 cm across) from the Kelly Mine, western margin of the northern Magdalena Mountains, southeast of Magdalena, western Socorro County, west-central New Mexico, USA.