Natural Opals

By Gemmological definition, a natural opal is an opal that has been mined or found and comes out of the ground, from a natural environment. At present there are two geological variations of Natural Opals. (See discussion of Geological Occurrences). We have natural opals that occur in or are associated with rocks that are of a sedimentary origin, and natural opals that occur or are associated with rocks of a volcanic origin. This distinction is difficult and eventually Scientific investigations will become much more definitive. However for now these two ‘environments’ provide us with a suitable distinction.

WHY DO WE SEPARATE OPAL INTO THESE TWO GROUPS?

The reason is based in the reaction or diagnosis of opal as a gemstone. It has been reported that opal has a range of refractive Index (RI) from approximately 1.37 - 1.47 (GIA (Gemological Institute of America) Gem reference Guide 1995) and is usually about 1.45. The Specific Gravity or density is related to be from approximately 1.25 - 2,23 and is usually about 2.15 for non-porous or non-absorbent varieties of opal (See discussion on different types and varieties of opal under the tab for opal nomenclature-hydrophane).

Natural opals found in a sedimentary environment.

These opals usually range slightly higher in their Refractive Index, (RI) and although there is a degree of overlapping, usually the range is approximately 1.42 - 1.45. The same is related for Specific Gravity, (SG) in that the range is usually from approximately 2.10 - 2.15.

Natural opals found associated with or in a volcanic environment.

These opals usually range slightly lower in their Refractive Index, (RI) and although there is a degree of overlapping, usually the range is approximately 1.40 - 1.42. The same is related for Specific Gravity, (SG) in that the range is usually from approximately 2.00 - 2.10.

There are other gemmological characteristics that support this separation as you can see in the appropriate table provided from “The Australian Gemmologist” and entitled “The unique attributes of Australian precious opal Volume 25, Number 6&7 (2014) or the article reprinted January 2018. Note that the article can be downloaded from the Gemmological Association of Australia (GAA) website.