Bismuth

Hardness: 2.25 on Mohs scale; Color: Silver-white (native), Iridescent rainbow (lab-grown); Luster: Metallic; Crystal structure: Trigonal/Rhombohedral (Hopper crystals); Cleavage: Perfect; Specific gravity: 9.78

Formed in hydrothermal veins and pegmatites. While it occurs naturally, specimens with this stair-step 'hopper' shape and rainbow color are lab-grown through a process of melting pure bismuth and allowing it to cool slowly so the edges grow faster than the centers.

Used in metallurgy (low-melting alloys), cosmetics (bismuth oxychloride), pharmaceuticals (Pepto-Bismol), and as a lead substitute in shot and fishing sinkers. Highly popular as a decorative collectors' specimen.

Bismuth is one of the few substances (like water) that expands when it freezes. It is the most naturally diamagnetic element and has unusually low thermal conductivity. For a long time, it was thought to be stable, but it was discovered in 2003 to be slightly radioactive, though with a half-life a billion times longer than the age of the universe.

Identify in the field by its high density and silver-white color; however, the rainbow 'hopper' crystals shown are always man-made. Look for the distinct laboratory-perfect geometric 'steps' and brilliant iridescent oxidation caused by thin-film interference.