Each type was formed underground in a slightly different way and as a consequence of this the mining technique varies depending on the type of opal.

Mining Techniques: Queensland Boulder Black Opal
As the name suggests Queensland boulder opal is in fact found in boulders. These boulders may be formed in an “opal level” or may be simply distributed randomly in the sandstone levels . Due to this unpredictability of boulder location unique mining techniques are employed in this type of opal mining.

In the earlier days of the modern period of machine mining in the boulder fields the mining technique was simply to remove all the overburden above the boulder carrying sandstone level . Once it could be determined at what level in the ground the boulders were running, the stripping of overburden could be a very quick earth moving operation, (occasionally even at night since there was no chance of missing opal in overburden). Sometimes explosives are used to blast the top of the ground as it was found to be too hard to rip with a bulldozer.

After the overburden has been stripped off the large bulldozers will then only remove a small layer of boulder bearing sandstone while “opal checkers” looked through the removed dirt and windrows looking for boulders . Once a boulder was found it was checked externally for any sign of opal , if none was found the opal checker would then break the boulder with a pick and hammer so as to inspect for opal contained within seams within. If any opal is found then the boulder is sent to the cutting shop for further processing .

Current mining techniques have been varied as it was found that simply removing all the dirt from a potential opal area to be too expensive and also necessitated lengthy restoration to replace the overburden after an area had been checked. Large excavators now do the majority of the boulder opals mining since these machines could quickly dig vertically down to the boulder bearing sandstone and clay levels. These levels are then checked for opal bearing boulders . After mining a given area the excavator simply continues to dig new overburden from an adjacent piece of ground and fills the previous hole with the new overburden . This technique is referred to as “strip mining”.



Mining Techniques: Black and Light Opal Mining Techniques
Black opal mined at Lightening Ridge and white opal mined in South Australia's opal fields is known as seam opal or nobbies. It differs from the Queensland fields in that it formed free in the ground in or around clay levels.

Mining techniques also vary , usually mining work is underground with shafts or drives . This suits the type of ground as opal is found in more concentrated areas , thus the need to move large areas of overburden is not necessary and smaller more cost effective techniques are used.

Shafts are dug into the ground vertically down to the “opal dirt” level , commonly a large drill is used and a hole could be dug to a depth of 20~40 feet.. Once the vertical shaft has been dug a further tunnel is dug horizontally to follow along the opal level , this is known as a drive.

Usually a tunnelling machine or some form of mechanical help is lowered down the shaft and assembled at the bottom and heads in the most likely direction looking for opal nobbies . If no opal is found in a given direction then another could be tried . If an area between 2 drives looks good , then this area can also be excavated forming what is known as a ballroom. Overburden and excess dirt in the drives is removed by giant vacuum cleaners known as blowers. It is then loaded into trucks and dumped if it is overburden and taken for checking or puddling if it is pay dirt.

In Lightning Ridge the pay dirt is mixed with water in old concrete agitators so as to clean excessive dirt off and expose potential opal nobbies, this process is known as puddling. The nobbies that are washed are then checked for any quality opal.