Fire color is the
term for the color flashing out of the opal and caused by the optical phenomena
called diffraction. Since these colors are produced by light, they can be all
the colors of the rainbow.
There is an interesting
physical phenomena in fire color. Some splashes of fire change color as you
move the opal. In effect, you are moving up an drown the color spectrum as the
angle of light changes. Other splashes of fire keep the same color but move
across the surface. These color changes are what make opal so fascinating.