How Opals are Formed

The stunning colors of a precious opal speak of its rarity. Precious opal gemstones are not found in many places in the world. Australia is home to 95% of the world’s deposit of opal. The special and specific conditions that led to the formation of opal are still being debated today by geologists.

One of the leading theories is the deep weathering model that contends that the process of opalisation began in Mid-Tertiary Period (around 30 million years ago). During this time, rocks underwent major weathering, which allowed ground water to seep through the weathered rock and resulting mud and sand. Water could have also originated from the Earth’s core by being hydrothermally driven upward. The water then proceeded to dissolve minerals and chemically alter dead plants and animals found in the weathered material. The chemically altered plants and animals formed large amounts of dissolved silica. As groundwater levels increased and decreased, the silica solution was able to seep through the ground. Eventually, the silica water solution would become trapped in the nooks and crannies of the rocks. Over several million years, the water evaporated, leaving behind first a silica gel and then hardened opal. The speed of evaporation determines the value of the opal, as too fast an evaporation results in cracks. Most of the time the opal formed is colorless. However, other times the process results in a brilliant precious gemstone.

The colors of precious opals result from the negatively charged gaps between the silica spheres. If the negatively charged gaps match a wavelength of visible light, the opal then has a “fire.” Opals with red fire are extremely popular because since red has the longest wavelength, a rotation of the opal allows viewers’ perspective of the negative areas to decrease; thusly allowing other colors to be seen. Red opals are capable of exhibiting all the colors in the spectrum.

There is still controversy over the exact details of opal formation regarding the timing of opal formation. The dominant viewpoint that formation occurred by the gradual deposition by layering of opal through millions of years has been challenged by new research. According to some scientists, opal could have been formed in two to three million years through a complex ion exchange process. Another detail that supports a quicker opal formation process is the discovery of aerobic microbial remains in opal. Studies have fixated on the possibility of these organisms aiding in the opal formation process. This would then suggest a quicker opal formation process in the Early Cretaceous Period, around 100 million years ago. Nevertheless, the radiant appearances of opal along with the experts who study opal formation speak to its distinctive, valuable qualities.