Australian Opal colour Patterns and how they form

I would like to have a closer look at the beauty of Australian Opals and specifically how they show different colours and patterns.

Opal is a basically Hydrated Silica that formed from an ancient ocean that covered the inside of Australia around 80-120 million years ago. As the ocean water evaporated and dropped underground into the ocean bed the silica that was in the ocean water was left and started forming a gel that over millennia hardened and formed Opal as we know it today. 

Opal when magnified looks a lot like a baby/toddler ball playpen with lots of circular balls stacked on one another , this can be orderly stacked in perfect alignment of randomly irregularly stacked. The neat arrangement of the silica spheres plays a large role in the strength and vibrancy of colour we see when looking at an Opal, this is due to light being pushed through a narrow space and causing diffraction of light that breaks light into different colours. The better aligned and formed the spheres the stronger the colour and the more random and misformed the paler or whiter the stone. This is why top colours such as my example attached is so rare and costly as it takes very perfect conditions in nature to have the spheres form perfect and neatly arranged.

Colour largely comes from the size of the actual spheres of Silica. The larger the sphere the more towards the Red colour spectrum it will show and smaller toward violet Blue colours. This is another reason the Red colour in Opal is more sought after due to the rarity as more stable conditions in earth is needed or millennia to formed larger spheres hence less material available. The base colour of the Opal influenced by the geographic area in Australia also plays a role; the darker the more contrast to the Opal hence more colour. This usually makes the Black Opals more sought after , see attached picture, due to the black contrast and this is evident when Doublets and Triplets are made as black cement is normally used to enhance the appearance.

Patterns is one of the main distinguishable factors to identify a natural from a synthetic as nature has little consistency in the appearance and more natural unevenness in the patterning. The pattern form due to different geological conditions in pressure and heat over millennia while the Opal forms some stages are irregular , some stable , some fast or slow. This causes the spheres of the Opal to become different sizes and stack differently that then makes all the colour and patterns come alive.

Truly an unique gem and an absolute feast to the eyes!