Nanotechnology and Gold - particles are are ruby red in color

The Heat Is On: Gold at the nanoscale
wow. Interesting article describes the physical characteristics of gold at the nanoscale.

Bulk gold has a familiar yellow color, which is caused by a reduction in the reflectivity of light at the blue end of the spectrum. However, if we subdivide the gold into smaller and smaller particles, there comes a point at which the particle size becomes smaller than the wavelength of incident light. New modes of interaction between the radiation and the gold become prominent, in particular interactions involving electronic oscillations called surface plasmons. When the particles of gold are small enough, they are ruby red in color. This coloration is due to the gold particles strong absorption of green light, corresponding to the frequency at which a resonance occurs with the gold.