If the electrons cannot absorb any energy from the light, then the radiation passes through unmolested, so the material appears to be transparent. However, if the electrons can rob the light of any energy, then the light is dimmed, obscured or absorbed. As a result, the material appears more or less opaque.

It was not known until this century why diamond and graphite, both made from carbon atoms and possessing the same number of electrons are optically so different. Scientists now know that the crystalline structure of a substance determines how tightly its electrons are held by the atoms and chemical bonds in the material.

At the wavelengths that humans can see, light does not have enough energy to excite the tightly held electrons in diamond, so diamond appears transparent to us. Impurities in the diamond, contribute electrons that can absorb light at characteristic wavelengths, to give the diamond varying hues of color. For example: pure diamond appears transparent, nitrogen in diamond causes a yellow to orange tint, and boron causes a blue coloration.

Some of the electrons in graphite are loosely held, so they can absorb and reemit (reflect) the energy of the light. The electrons' interactions with light give graphite a metallic luster.