Opal is among one of the most rarest gemstones on the planet, including itself amongst Diamonds and Emeralds.
There are many factors that play in a Opal's rarity but in general the Precious Gemstone is rare in of itself. Only because it is formed from a solution of silicon dioxide and water.
As water seeps through the sandstone, it picks up tiny particles of silica. It takes over 2 million years for this solution to flow into cracks and voids in sedimentary rocks as well as volcanic areas inland Australia to then harden and form into Precious Opal that has stunning play of colour.
Silica is one of the most common minerals on the planet, but precious opal is very rare because of the long natural processes that take to create it.
Opal is a miraculous rare stone overall, but there are more specifics that define its rarity amongst other Opal gemstones. To dig in further about these specific factors in defining rarity and value between Opals, please visit
https://australianopaldirect.com/how-much-is-opal-worth/