Pyrite
The first thing you may notice about these pyrite specimens is their golden luster. But look more closely to see the striking diversity of crystal shapes. These pyrite crystals are all built of iron and sulfur atoms linked in the same cubic pattern. But different growth conditions created distinctive external variations on one basic atomic pattern. Cubes tend to grow under low temperatures from solutions with low concentrations of iron and sulfur. Octahedrons and pyritohedrons result from higher temperatures and more concentrated solutions.

Calcite
Over 2,500 distinct crystal shapes of calcite have been observed, more than for any other mineral. What accounts for this incredible diversity? No one is sure. All calcite crystals have the same arrangement of calcium, carbon, and oxygen atoms in a pattern with three-fold symmetry. But research suggests that impurities, along with differences in temperature and composition, cause the crystal faces to grow at different rates—creating many variations on one basic shape.