The brilliance of the play of color is the most important factor.
The colors seen and the pattern of the colors will also influence value.
Black opal, with a black to dark gray body color, has the most brilliant colors and is the most valuable.
White and milky opals tend to have more diffused colors due to the light background color, thus making these qualities less expensive.
Opal with red fire is the most valued because the opal that shows red will also show the whole spectral colors when rotated.
The pattern of the play of color also influences value. Large flashes and broad patterns are rarer and more valuable than small pinpoint patterns.