Opal - (Australia, Ethiopia) Precious opals display 'opalescence' (spectral color, irridescence that changes with the angle at which the gem is viewed). They can be lighter in color (white or yellow) and contain flashes of color or have a darker color to the backgound with with obvious color (black opals). Black opals are more rare and can be much more expensive. Lighter opals can be darkened with radiation. 90% of opals come from Australia. Common opal is generally opaque with no or very little flash.