In general, Opals are being categorized on the basis of the intensity and extent of fire they hold. A double-sided opal has a strong fire extending throughout its surface and is visible from both the side and all angles. An opal with fire exhibits more prominent play-of-colors on the upper surface and less on the inner surface.
However, both these variants of opal are considered worthy in the astrological drive, but as the lower side of any gem directly touches the wearer’s skin; an opal with double side fire is considered better.