Boulder Opal formed around 20-30 million years ago in iron rich sandstone laid down during the Cretaceous period 65 to 140 million years ago. This was the age of the dinosaurs. A large Inland Sea called the Eromanga Sea covered a large proportion of Australia. This was the start of this beautiful Australian National Gemstone. During this period of extreme weather conditions when the climate was wet and warm, a of deep weathering, chemical reactions and pressure enabled the silica rich deposits to form opal.
Boulder Opal is formed from layers of minute silica spheres that are uniformly layered. Between these hydrated silica spheres, light is diffracted in the beautiful rainbow colours of the opal. The resultant colours are in accordance to the size of the silica spheres and how closely packed they are. For example the smaller spheres create the blue, which is more common, and the larger spheres create the rarer orange and red opal, this rarity is reflected in the price. It is estimated it can take up to 5 million years for just 1 millimetre of opal to form.