How Is Opal Formed?

Derived from the Sanskrit word ‘Upala’ meaning ‘precious stone’, opal is a stunning iridescent gemstone that changes its color in different lights. It is the birthstone for the month of October and is often used to mark 14 years of a happy marriage. The stone’s color is a magnificent mystery and it can display every color of the rainbow, ranging from green, blue, yellow, aqua, pink to basically any other color that you can imagine. This happens due to the phenomenon of ‘Diffraction’, different opals diffract light differently to create moving spectrums of color across the face of this gemstone.

In 1892, a novel published by Sir Walter Scott gave this stone a bad reputation as his story featured an heiress with demonic powers who wore rainbow stones in her hair. Although entirely fiction, his novel reduced opal sales all across Europe because people started considering these stones as markers of dark forces. These superstitions no longer hold any value and opals are now associated with peace, tranquility and are considered a symbol for hope. Their rainbow-like play of color is often considered as a marker of their positive properties. These are unique, rare gemstones with unmatched beauty and jewelry made from this stone is highly demanded all across the world.

Formation:

Unlike, most gemstones, real opals exist in non-crystalline form and is composed of amorphous lumps and ball of silica with water. Under ideal conditions due to the gravity of the Earth’s core, silica rich solutions get embedded within the deeper layers and voids of the earth’s surface creating spheres of silica. This solution deposits at a rate of one centimeter thickness at a depth of nearly forty meters in approximately 5 million years. As the process continues and the silica spheres start reaching uniform size, an opal comes into existence.

Occurrence:

These rainbow stones usually occur in regions that must have contained voids or porosity in the past to allow deposition of silica rich solutions to take place. Weathering processes and volcanic eruptions can create a wide variety of cracks and voids in sedimentary rocks. These areas serve as appropriate molds for the deposition of secondary minerals like Opals. Opaline silica not just deposits itself in larger cracks but also fill the spaces in silt and sand to cement grains and particles of the sand together. However, this type of opal deposition is not considered precious as it is simple opalized sandstone. The ‘common opal’ of ‘Potch’ as called by miners is the opal that has zero display of colors.

Composition and variations:

The composition of all opals is variable giving each its unique properties and color. These rainbow stones are composed of hydrated silica with nearly 1% – 21% water content. The numerous variations of Opals depend on a large number of factors, the most important of these are the climatic conditions. Changing environment creates a rising or falling water table allowing silica to accumulate in any solution. An acidic environment mostly created by the presence of microbes is also necessary to give rise to silica spheres.

Depending on the developing conditions there are several classes of opals. The most rare and expensive of these gemstones is ‘Precious opal’ which has 6% to 10% water content and measures 5.5 – 6 on Moh’s hardness scale. The silica spheres of 150 – 300 nanometers in diameter are arranged in a hexagonal fashion within the lattice.

Opals that do not show a display of color are termed as ‘Common Opals’. These include resin opal: a honey-yellow opal with a resinous appearance, wood opal: developed when silica replaces the organic materials within wood and Sinter; also called geyserite opal most found near geysers and hot springs.  The least valuable and relatively less rare type of common opal is the Milky Opal. Mostly sourced from Southern Australia these are pale white in color. Their lighter body tone contributes to their lack of vibrancy and display of color.

A type of real opal that is highly demanded because of its beauty and luster is the Fire Opal. Within the depths of ancient volcanoes when silica-rich lava deposits in the seams and hollows it gives rise to these sun-bright droplets of lava. Under the high pressure and large amount of heat, water gets trapped within the surrounding lava to make these lumps of fire. Found in New Mexico, they have warm body colors ranging from yellow, orange to deep red. These stones often display flashes of bright green light.

Formation of colors and patterns:

All opals contain silica sphere but it is the uniformity and orderliness of these spheres that separates opals. The arrangement of these spheres is what determine the colors, size and patterns of an opal that comes into existence. The more uniform the spheres the brighter and more vibrant the color of the opal formed. Different sphere sizes cause diffraction/refraction differently, causing the light to scatter at wavelengths of different colors. While the small-sized spheres tend to bring out the blues, the larger spheres exhibit shades of red.

On a molecular level, minute particles of silica arrange themselves in 3-dimensional grids with different spherical arrangements. It is the spaces between the spheres and their variable size that gives rise to various patterns observed amongst Opals.

Synthetic Opals:

Above we’ve described how Opals come into existence in mother nature, but man has found ways to artificially create Opals of their own. The ‘Gilson Opal’ and ‘Slocum Stone’ are created by embedding glass with fine tinsel-like threads of color. Later their beauty is enhanced by treating them with several chemicals, heat and strong radiations to add character and color. Synthetic opals are much cheaper as compared to natural ones, so if you’re someone looking for an attractive piece but at a lower cost, this option is right for you

Our Final Thoughts!

Opal is indeed a stone of much value and can help make stunning jewelry pieces that are bound to make you stand out from the crowd. However, if you’re looking to invest in this gemstone, it is essential to remember that there are many imitations and fake alternatives that claim to be an Opal in the market. Make sure you always purchase your opals from a reputable place and seller to ensure that you pay the appropriate price and aren’t ripped off.