Opal Formation

Opals are beautiful stones used for jewelry and small sculptures. Many of them are white or clear, but some of them have a gorgeous array of colors.

Chemically, opal is similar to glass, quartz, and those little silica gel packets that are meant to keep the contents of small boxes dry. Opals contain water in various amounts. In fact, it is possible for the stone to dehydrate, which usually will cause it to crack. According to Gbjewelers.com, the gem quality opals would have hardened slowly and contain less than 10% water. An especially interesting thing about opals is that they are not crystals. Instead, they are composed of a matrix of tiny negatively-charged silica spheres.

It is generally understood how crystals developed under various conditions, producing the many kinds of crystalline stones. However, the formation of non- crystalline stones generally – and opal particularly – is not entirely known. According to the New South Wales Department of Primary Industries and the Lightning Ridge Tourist Association website, there are three main models or concepts which are believed to be plausible descriptions of the formation of opals: “deep weathering”, “syntectonic/Pecover”, and “microbe.”

*Deep Weathering

The Deep Weathering model posits that eroded or weathered sandstone is the source of the silica from which opal is made. This silica would have combined with groundwater to produce a silica gel, which was trapped in the rocks and later dehydrated to form the opal.


The Syntectonic model is similar but, in chicken-vs-egg fashion, posits that the silica was one of the many minerals in mineral-rich subterranean water pushed toward the surface along breaks in the rock bed. The opal seams then formed as the water leached away.


In many opal-rich areas, there are also fossilized bacteria found in the rock. It is speculated that the bacterial may have contributed to a set of conditions conducive to the formation of opal by causing a chemical “weathering” of the surrounding rocks.

Discover News reported in 2008 that a NASA probe discovered some opal deposits on Mars.  Because water is a part of opal formation, the presence of opal indicates that there had been water on the planet. This had already been known, because there are two older kinds of mineral deposits which also indicate the presence of water. As the youngest deposits, the opal tells the NASA scientists that water was on Mars for a longer period of time than was previously believed.

There is doubt as to the precise manner in which natural opals formed, but it is highly likely that the formation included the dehydration of a silica gel.