Sometimes the hard, organic remains of plants such as wood or cones, or the bones or shells of animals are buried in lava or sediments before they can decay. Such burial restricts oxygen supply, and decomposition processes slow to a snail's pace. Silica laden waters can, ever so slowly, fill and replace any cavities or structures that are present with agate or opal, preserving a replica of the original form in solid rock. Many fossils are the result of this process, known as petrifaction.
Image courtesy of Las Vegas Jewelry and Mineral, opalized clam fossil (opal solution filled the cavity of the clam shell and solidified before the shell decayed. Remnants of the fossil shell were then cut and polished away, revealing a perfect "cast" of the original shape

Opal formation in the fossil

The process by which an organism is completely replaced by minerals is called petrifaction. The best-known example of this process is petrified wood, as seen in the Petrified Forest National Park in Arizona. Trees in this area were buried in mud and sand that contained the mineral silica. Ground water carried this dissolved mineral into the trees, where it replaced the wood cells so completely the trees became hardened opal.

The Different Colors of Opal

Opals come in a vast range of natural colors and shades. Black opal is more valuable than crystal Opal, and crystal Opal is more valuable than white or milky Opals. The more colors in an Opal the more valuable the gemstone will be. The dominant colors in an Opal an Opal have value in the order of red, orange, yellow, green, blue, indigo and violet.

Tectonics and Climate

This interpretation would be consistent with the observed increase in trade wind strength, continental aridification (Atacama), and productivity as indicated by a strengthened increase in dust, organic carbon, and biogenic opal accumulation rates

Opal Rock Cycle

An opal is considered to be a mineraloid, meaning that it has all of the properties of a mineral-- solid, inorganic, naturally forming, has a chemical composition-- but does not demonstrate crystallinity. Other examples of mineraloids include obsidian, an amorphous glass (that is not a crystal).

How the Earth's layer affects Opal rock cycle

When magma rises through cracks and cools slowly underground, it forms igneous rocks composed of minerals with fairly large crystal sizes, these are known asintrusive igneous rocks. When the magma erupts onto the surface, as through a volcano, it is termed lava, and depending on the rate of cooling, the extrusive igneous rocks which form have medium to very small mineral crystals. Some lava cools so rapidly that it forms an amorphous material without a crystalline structure. Granite and basalt are examples of larger and smaller grained igneous rocks, respectively, and obsidian (volcanic glass) is amorphous.

Once the igneous rock is on the surface, the forces of erosion and weathering produce smaller particles which accumulate on the surface, and/or are moved by wind and water. As time proceeds, layers of these sediments build up (on land or under water). The pressure from upper layers causes compaction in the lower layers along with various chemical and physical changes (lithification), which lead to the creation of sedimentary rock.

Evaporation is an alternate factor which also produces sedimentary rocks, as when dripping mineral-laden waters leave behind stalactites or stalagmites. Likewise, surface or subterranean waters carrying dissolved minerals may evaporate or precipitate those minerals within the cracks in other rocks, or between rock layers. Sandstone and limestone are familiar sedimentary rocks formed by lithification. Opal and turquoise illustrate the evaporative mode of formation.

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Near Surface Environments (Tectonic plates and the Opal)

The presence of intrusive magma in a local region (contact metamorphism), or of tectonic plate interactions on a larger scale (regional metamorphism) puts the igneous and sedimentary rocks and minerals under heat and/or pressure which may cause changes in their chemistry and crystal structure. The result is the creation of metamorphic rocks. Thus is limestone turned into marble, sandstone into quartzite, and serpentine into nephrite jade.

As with most cycles in Nature there are sub-cycles and cross interactions. So, for example, sedimentary rocks which are subducted through tectonic action may melt and form magma which produces igneous rocks. Or metamorphic rocks, which have been uplifted and exposed at the surface, will erode to form sedimentary deposits.

Near surface waters, like rainwater, move down or up, through soil or rock, as the local cycles of precipitation and evaporation dictate. Such water has carbon dioxide from the air dissolved in it, which creates a weak acid solution (carbonic acid) in which many minerals are soluble. If the environment contains sandy soils or sandstone rock, then silica will be dissolved, and certain silicate gems such as aggregate quartzes, like agates, or amorphous opals may form as the water evaporates.

Earth's environment when Opals started forming

The Great Artesian Basin was once filled with the Eromanga Sea, a shallow, cold, muddy, stagnant body of water that at its peak flooded 60 percent of Australia. Rey discovered the formation of Australian opal was apparently due to an extraordinary instance of acidic oxidative weathering during the drying out of the central Australian landscape that followed the retreat of the Eromanga Sea from the basin starting about 100 million years ago. Such acidic oxidative weathering over a large region is unusual on Earth — the planet's surface is usually loaded with carbonates that neutralize acid — explaining why opal is rare elsewhere near Earth's surface.