This discussion for opal occurrences at present is a discussion of the occurrence of Precious Opal. Precious opal shows the phenomenon of Play-of-Colour which makes opal prised as a gemstone. A later discussion of ornamental Common opal (opal that does not show a play-of-colour) will follow in later discussion. For a long time Australia has been preeminent in the gemstone market for producing the worlds best opal gemstones and so it is fitting that Australian opal occurrences should head up the discussion regarding where opal is found and the geological environment for opal formation. So this discussion is separated into Australian opal occurrences and Foreign opal occurrences which produce precious opal.
In no way is this meant to remove the providence of the most historical opal occurrence of Slovakia, and European opal or to take away the importance of the most recent and very productive opal occurrence in Ethiopia, both of which will be described and give due diligence further down in on this page.
THE GEOLOGICAL ENVIRONMENT FOR OPAL FORMATION
There has been a lot of information published on where opal is found, what geological profiles are involved and where opal should be explored, or prospected for.
Much more information is available for discussion in other parts of the Opal Library. You are encouraged to read and investigate, criticise and comment on all of the relevant issues.
Rightly or wrongly opal formation and geology issues have been subdivided (and not by preference but convenience) into two ”geological environments” . The science is still under investigation, postulation and discussion.
In many recent publications there has been a divide for opal formation into two different environments. A “Sedimentary” based occurrence for opal and a “volcanic” occurrence for opal.
This is a convenient separation for opal both from a mineralogical and a gemmological viewpoint.
Opal from a so called “Sedimentary environment” has a higher Refractive Index and a Higher density (Specific Gravity) than those opals formed in a so called “Volcanic environment”. There are also other characteristics that are helpful in this separation.
It should be taken into context for opal formation that the opal occurrence is described by the host rock in which the opal is formed. and not related to the formation of the host rock.
To emphasise this relationship, the host rock formed first in the geological environment and the opal formed at some later time and through a different process.
The tables presented below a representative of precious opal formation and are given with references to the geological papers that discuss the characteristics of the regional geology. The “field” localities are not an extensive list, however are representative in the geographical areas.
