The October birthstone opal is a mineraloid that took millions of years before getting its shape. The constant heating of the silica and flow of the water through the cracks lead to its formation, giving wonderful Opal to the world. It comes in two different varieties, precious and common, which are determined by the play of colors in the light.They used to be quite rare, but after discovering the rich mines in Australia, they become pretty accessible to the modern markets. Today, they have a wide availability in Ethiopia, United States, Mexico, and Brazil.