Opal occurs in a myriad of forms, some of which are obvious to the naked eye, and others which do not appear in the least like opal. Most of us are familiar with precious opal, with its unique (in the inorganic world of nature) display of colours. The gem material itself occurs in a variety of patterns, and may also be found in various forms replacing small and large marine creatures, bones and wood. Associated with the gemstone in its natural occurrence is potch, which, while showing no play of colour, can occur in a variety of shades of light grey to black, with intermediate smoky, and occasional yellow or orange colours.

Many of these not only make attractive specimens for the collector, but can be cut and polished to make acceptable ornamental gem material. Most of these materials are composed of the amorphous form called opal-A. However some of the volcanic opals are composed of more crystalline material, opal-CT or opal-C.

Another form of opal-A is that known as hyalite. It most commonly occurs associated with volcanic rocks, usually deposited in vugs as a transparent colourless glass-like material. It is sometimes found in this form in upper zones of ore deposits. A further form of opal-A is found in some volcanic areas where there is (or has been) much thermal activity at the earth's surface. Large deposits of silica, known as siliceous sinter, or geyserite, may be formed. These may be found in areas such as Yellowstone National Park in the U.S.A., but perhaps the most famous example was the so-called 'pink terraces' at Rotorua in New Zealand. These terraces were destroyed by an earthquake in 1884.

Common opal, usually opal-CT, also occurs in a wide variety of forms and body colour, but does not show the play of colours found in precious opal. The reason for this is the fundamentally different microstructure - but more of that later.

The body colour of common opal is largely due to impurity elements, particularly iron. Many opals contain a substantial amount of iron oxide; one sample was found to contain more than 20% ferric oxide. These opals range in colour from pale to dark brown, rarely reddish, depending on the amount and nature of the iron oxide impurity.

Opals with impurities such as alumina, lime, or alkalies usually contain very little iron, and tend to be pale grey to white in colour. Other impurities, such as traces of copper or nickel, for example, may give them a green tinge. If the amount of impurity is small and the crystallites very small, common opal may be translucent and virtually colourless.

Many common opals vary in colour and pattern due to uneven distribution of impurities, and some of these also become useful ornamental stones. Particularly attractive are those containing dendrites usually of manganese oxide. The resultant fern-like growths can give striking cabochons for jewellery. Others, as with some types of agate, can give rise to landscape and other patterns.

Common opal also may replace organic materials, especially wood. It can preserve the woody structures in great detail. It is sometimes associated with chalcedony in this form.

All of the above can readily be recognised in a hand specimen as a form of opal. However there are numerous forms of opal which are less easily recognised. Many of these are derived from the skeletons of plants or animals.

Phytoliths are small, often needle-like remains derived from grasses and other plants and are to be found in soils and may sometimes accumulate in small deposits. Many trees incorporate silica into their cellular structures; the opaline materials eventually return to the soil on the death of the tree and decomposition of the wood.

Microscopic marine animals such as diatoms and radiolaria can have skeletons which are composed of silica. On the death of these creatures vast deposits of earthy opaline materials can accumulate.

Thus, we find soft or firm earthy deposits of diatomaceous earth, often mixed with clay; the latter is termed opal claystone, tripoli, gaize and opoka in various countries in which they are found. In this form it is an important industrial raw material. In the deeper parts of the ocean, these skeletons are still building up deposits, and undergoing changes which result in changes in their microstructure, but again, more of this later.