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Occasionally, when conditions are ideal, spheres of silica, contained in silica-rich solutions in the earth form and settle under gravity in a void to form layers of silica spheres. The solution is believed to have a rate of deposition of approximately one centimetre thickness in five million years at a depth of forty metres. If the process allows spheres to reach uniform size, then precious opal commences to form. For precious opal the sphere size ranges from approximately 150 to 400 nanometres producing a play of colour by diffraction in the visible light range of 400 to 700 nanometres.

Each local opal field or occurrence must have contained voids or porosity of some sort to provide a site for opal deposition. In volcanic rocks and adjacent environments the opal appears to fill only vughs and cracks whereas in sedimentary rocks there are a variety of voids created by the weathering process. Leaching of carbonate from boulders, nodules, many different fossils, along with the existing cracks, open centres of ironstone nodules and horizontal seams provide a myriad of moulds ready for the deposition of secondary minerals such as opal.

Much of the opal deposition is not precious. It is called “potch” by the miners, or common opal by the mineralogist, as it does not show a play of colour. Opaline silica not only fills the larger voids mentioned but also may fill the pore space in silt and sand size sediments cementing the grains together forming unique deposits, known as matrix, opalised sandstone or “concrete” which is a more conglomeratic unit near the base of early Cretaceous sediments.

The many variations in the types of opal depends on a number of factors. In particular, the climate provides alternating wet and dry periods, creating a rising or more importantly a falling water table which concentrates any silica in solution. The silica itself is formed either by volcanic origin or by deep weathering of Cretaceous clay sediments producing both silica and white kaolin often seen associated with the Australian opal fields. Special conditions must also prevail to slow down a falling water table in order to provide the unique situation for the production of its own variety of opal.

The chemical conditions responsible for producing opal are still being researched, however some maintain that there must be acidic conditions at some stage during the process to form silica spheres, possibly created by microbes.

While volcanic-hosted and other types of precious opal are found in Australia, virtually all economic production comes from sediment-hosted deposits associated with the Great Australian Basin. Australia has three major varieties of natural sediment-hosted precious opal – black opals from Lightning Ridge in New South Wales, white opals from South Australia, and Queensland boulder and matrix opal.
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Sometimes, under special conditions, tiny silica balls form in the ground. They settle down and create layers of these balls. It takes a very long time, around five million years, for them to stack up to a thickness of one centimeter at a depth of forty meters. When the balls are the same size and shape, precious opal starts to form. These opals have balls ranging from 150 to 400 nanometers that reflect different colors when light shines on them.

Opal is found in places where there were gaps or empty spaces for it to go. In some rocky areas, it fills only small cracks, while in other places, like sedimentary rocks, there are different kinds of holes made by weathering. Opal can also fill spaces between tiny grains of sand or silt, sticking them together. It creates unique deposits called matrix, opalized sandstone, or "concrete."

Not all opal is special or precious. Some are called "potch" by miners or "common opal" by scientists because they don't have colorful reflections. Opaline silica not only fills larger gaps, but it can also fill the tiny spaces between grains of sand and silt. This makes the grains stick together and forms different types of opal.

The type of opal depends on many things. The weather plays a big role, with its alternating wet and dry periods that affect the water level in the ground. This helps gather the silica needed for opal formation. The silica can come from volcanoes or from the deep weathering of certain clay sediments. The climate and other special conditions also slow down the water level, creating the right conditions for opal to form.

Scientists are still studying how opal is formed, but some think that acidic conditions, possibly created by tiny living things called microbes, are necessary for the process.

In Australia, most of the valuable opal is found in sedimentary deposits associated with the Great Australian Basin. There are three main types of precious opal found there: black opals from Lightning Ridge in New South Wales, white opals from South Australia, and boulder and matrix opal from Queensland.