One of the two governing factors controlling quality. This factor is said by many to be very difficult to judge particularly for inexperienced persons. There is a very simple method of assessing this. Forget about relative brightness of opal colours, think of a paint colour chart. The bright colours are clearly evident from the normal or dull colours while fluoro colours represent brilliant to vivid colour. If you can judge a paint chart, you can judge an opal's brightness, or enlarge each of the brightness photos in turn until a match is found.

An Opal's brightness has an enormous affect on it's value, all finest gem quality Opals have high levels of brightness, two Opals with the same colours and pattern but showing different levels of brightness, say, brilliant for one and normal for the other could have 50 % difference in price, try it for yourself, value a stone then change the brightness levels, the new values will be very different (see Operating instructions for this method).