Know Your Opals
Australian opals are mostly sedimentary. They have low porosity and are therefore much more robust than their hydrophane (thirsty) cousins of volcanic origins found elsewhere— notably in Ethiopia. When it comes to the quality of colour play, durability and beauty, Australian opals have no equals.
In simplest terms, Australian opals can be divided into black, dark and light/white opals, depending on how dark or light their body tone is. It is also customary for Australian boulder opals to be considered an opal type. Technically, Australian boulder opals should be described as black boulder opals, dark boulder opals and light or white boulder opals.
It is against opals’ body tone, white, light, dark or black, that opal colours play out. An opal may show all the colours of the rainbow, or focus on just one hue with different saturation which produces a more subtle play of colour.
There is no universally agreed opal nomenclature because all opals are unique. Yet, opal professionals may divide opals based on the following characteristics:
Precious or common
Precious opals possess one quality that differentiates it from all other gemstones: play of colour, which has opal colours shift and change as the gem moves in relation to the light source. Common opals are chemically the same as precious opals, minus the play of colour.
Solid or composite
Composite opals, such as opal doublets or triplets, are made with a slice of opal glued to other material to make the price more approachable. Solid opals may be pure opal or opal cut together with its natural host rock.
Treated or untreated
As is the case with all other gemstones such as diamonds, rubies, emeralds or pearls, opals may be treated to enhance their desirable characteristics, such as brightness or play of colour.
Sedimentary or volcanic
The geological process of opal formation determines their porosity and water content. Sedimentary (most Australian) opals have significantly lower water content than volcanic (most Ethiopian) opals, so they are more durable.
Majority of opals are defined by their origin, which carries assumptions as to these opals’ qualities. Some well-known opal fields in Australia are in Jundah-Opalville, Winton, Lightning Ridge or Coober Pedy.
Understanding the various types of opal and their provenance carries implications for their wearability, durability, care and value. So, it pays to do your homework.
Opals can also be found in North and South America, Europe and Africa. They can be produced in a laboratory (with mixed results) and made into composite stones known as doublets and triplets.
The natural untreated Australian opal’s tightly packed molecular structure makes it significantly more durable and valuable than a colourful and impressive-looking hydrophane opal. The latter’s thirsty nature lends itself more easily to chemical treatments to increase its colour intensity – mainly to imitate a quality Australian opal.