Color is caused by the absorption of certain wavelengths of light within the material. Metals, including chrome, iron, cobalt, copper, manganese, nickel, and vanadium, absorb certain wavelengths of light and thus cause coloration. Measurement of color is very subjective, and not usually a good diagnostic for identification because many different materials may have similar colors.
  Refraction of light through materials change. The velocity of light through a material is inversely related to its index of refraction. For a vacuum, n = 1.0. Most minerals range between 1.4 and 3.2. This property will not be tested.
  Birefringence is when there are two refractive indices for cross-polarized light entering a material. If one ray enters, two rays emerge (the ordinary and extraordinary). If the material is rotated, the ordinary ray stays still and the extraordinary ray traces a circle about the former. Calcite is a famous source of this effect. Sodium Nitrate exhibits similar abilities. The two exitting rays are always polarized at right angles with respect to each other.
  Dispersion is the spread in refractive indices of light through a material when its wavelength varies from red to violet. The larger the dispersion, the greater the colors of white light will be separated upon exiting the material. Diamond's dispersion is particularly great, so a play of colors is exhibited. This property will not be tested.

Color Change: Two types...

Pleochroism is the effect when a material appears to have different colors when viewed in different directions. Amorphous and cubic materials show no pleochroism. Dichroism is an effect where the material exhibits two colors. Trichroism is an effect when the material exhibits three colors. Iolite (cordierite) is a mineral that is dichroic changing colors between dark blue and nearly colorless. None of the grown crystals exhibits pleochroism.

Another color change is due to the presence of artificial or natural light (or often flourescent versus incandescent light). Nickel sulfate shows this kind of color change reminiscent of the near-priceless gemstone alexandrite.

  Polarization of light does occur in some materials. In nature, it occurs strongly in tourmaline. Placed between a "polarization sandwich," sodium chlorate will permit different colors through as the angle between the polarizers changes.
  Cleavage occurs when a crystal breaks along certain planes more easily than in any other direction. In nature, a distinction is made between perfect and imperfect cleavage. Perfect cleavage (as in the case of flourite, calcite, and diamond) will always break along a cleavage plane. This makes cutting diamond more chalenging, because the angles needed by the gem cutter might not match cleavage planes. Imperfect cleavage (quartz or beryl) can break in direction. Broken shards of quartz look suspiciously like glass. A shattered octahedron of flourite, however, will produce many smaller pyramids, octahedrons, and tabular octahedrons.

Cleavage of Nickel Sulfate Hexahydrate.

  The Piezoelectric Effect is when a crystal that is compressed produces a charge-flow and voltage drop across the oposite poles. Rochelle salt and natural quartz exhibit this effect. Working the other way, piezo devices can be expanded by applying a voltage across the oposite poles. Piezo horns and tweeters work this way. Also, piezo devices are used to position scanning tunneling electron microscope probes. Some microphones use piezo devices to convert sound to voltage.

 Crystals that were Grown