All the continents on earth produce common opal. Only very few countries produce opal of gem quality. Common opal is referred to as opal with no color play. Most common opal can be grey, white to even black opal. Black opal is only valuable when it has colors of the spectrum.
Silica spheres are a significant contributor to the way opal shows color. The straighter and neater the silica spheres the better the color will be. The larger the silica spheres are, the redder the color. If the silica spheres are unevenly stacked, you have common opal, with no color. The light has nowhere to go and stops before it can produce and color play out of the opal.
Queensland boulder opal produces large quantities of boulder opal in the seams and veins it is found in. Only tiny pockets of boulder opal are of premium quality. An opal miner can be searching for years finding only common opal in the hope that they will find the gem boulder opal.
Common opal is a great starting place for lapidists who are learning how to cut opal. The risk of losing good color is not that risky. Getting a polish on opal is an art and can take a long time to perfect, so common opal has its uses.
Common opal is also used as a trace to find the better quality opal. If you have an opal bearing level with common opal, the chances of finding better color have been magnified.
Some common opal is used for backing on doublet and triplet opal as long as it is clean with no sand.