Minerals grow in specific shapes, and usually crystallize into one of six crystal systems. The axes of the crystal, the angles at which the axes intersect, and the degree of symmetry define each system.
Isometric -- Also called the cubic crystal system. Crystals are usually shaped like blocks, with similar and symmetrical faces. The crystal has three axes of symmetry, all at right angles to each other, and all of the same length.
Tetragonal -- Typically, the crystals are shaped like four-sided prisms and pyramids. Each crystal has three axes, all perpendicular to one another. Two axes are the same length and lie on a horizontal plane. The third axis is not the same length and is at a right angle to the other two.
Hexagonal -- These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three.
Orthorhombic -- These crystals are short and stubby. Each crystal has three unequal axes, all at right angles to one another.
Monoclinic -- Crystals are short and stubby with tilted faces at each end. Each crystal has three unequal axes. Two axes lie in the same plane at right angles to each other. The third axis is inclined.
Triclinic -- Crystals are usually flat with sharp edges, but exhibit no right angles. Each crystal has three unequal axes. None are perpendicular to one another.