Kyanite can be identified by its blue color, or by the unique quality that it has a different hardness in two directions. Along the length of the crystal, kyanite is much softer than on its sides.
The distinctive, ‘hoppered’, shape of a Bismuth crystal results from a higher growth rate around its outer edges than on its inside face. The higher rate of growth on the edges forms a crystal which appears to be partially hollowed out in a rectangular-spiral stair step design.
Generally speaking, thin, flat plates happen when a crystal’s unit cell (the smallest repeating block of atoms) is short in two directions and long in the third, like a tall, skinny box. Wulfenite is no exception, with the tall direction more than twice as long as the other two.
Quartz (SiO2) is clear and colorless, but a little bit of other elements in the mix can change the color drastically. Smoky quartz includes a bit of elemental silicon, making it black, and citrine’s yellow hue comes from iron contaminants.
Hemimorphite, Zn4Si2O7(OH)2·H2O, is named for the crystallographic phenomenon it exemplifies. A hemimorphic crystal is one whose opposite faces are symmetrically inequivalent.