Opal, and Black Opals, Lightning Ridge Australia | All prices in Australian dollars

About Opal

WHAT IS AN OPAL?

Opal is Australia’s National Gemstone, and we are proud to state that over 95% of the world’s precious gem opal comes from Australia. Opal is a type of gem that consists of hydrated amorphous form of silica (SiO2.nH20) which were naturally formed over millions of years ago. In simpler terms, an opal is a solid gemstone without a clearly defined shape or form that is solidified from chemical silicon dioxide. There are two types of opal - precious and common.

Precious, valuable opal exhibits the phenomenon known as “play of colour”. Opal’s optical effects are produced by the diffraction of white light passing over and through the orderly arranged spheres of silica within an opal’s microstructure. Refraction and Internal Reflection also contribute to an opal’s unique effects. In essence, it is the size and arrangement of these silica spheres coupled with its transparency that determines the variations of colours present in an opal. Colours include clear through to white, grey, black, brown body tones; and red, orange, yellow, green, blue, magenta, pink, green and purple play of colours. Of these colours, red play of colour on a black body tone is the most valuable and sought-after combination due to its rarity and intense visual effect. True quality “Red on Black” is found in Australia, specifically Lightning Ridge.

On the contrary, common opal does not display any play of colour. The colour and transparency may also be influenced by the type and level of impurities. For example, black coloured potch has been identified as containing higher levels of iron and carbon compared to its lighter varieties. It is given the name “common” because it can be found in more abundance and does not attract significant commercial attention yet.


Additional information on opal         
Mineral: Hydrated silica
National gemstone: Australia
Colour: All colours
Mohs hardness: 5.5 to 6.5
Birthstone: October
Fracture: Conchoidal
Specific gravity:1.9 to 2.3
Transparency:Transparent to opaque        

 

HOW IS OPAL FORMED?

There are many theories on how opal is formed and much evidence to support each theory.  The three most popular theories include: The Deep Weathering Model, The Syntectonic Model and The Microbe Model. Most scientists agree the it is an ongoing weathering process requiring a silica source, an environment hostile enough for the silica to dissolve from the host source, water to transport it and the electrolytes required to provide the right environment to dissolve and later precipitate the silica as an opal.

WHERE CAN OPAL BE FOUND?

Opal can be found around the world; however, majority of the highest quality is predominantly found in Australia. Other countries include Brazil, Ethiopia, Honduras, Indonesia, Mexico, Peru, Canada and the United States.  It is documented that precious opal was mined in Hungary 2000 years ago, but it is believed this source is now completely depleted. Varieties of opal found in these regions are outlined below:

Australia

  • Solid opal (black, dark or semi-black, light and crystal)
  • Boulder opal
  • Matrix opal
    • For example, Andamooka opal found in limestone, sandstone or quartzite rocks in South Australia.

    Brazil

    • Solid sedimentary opal (light and crystal opal with varieties of pastel colours)

    Canada

    • Solid common opal

    Ethiopia

    • Solid fire opal (orange, yellow and reddish body tone, sometimes with a little play of colour)
    • Hydrophane opal known as Welo opals (light to white, crystal and black) found in stratified volcanic rocks such as rhyolite and ignimbrite

    Honduras

    • Black, matrix opal found within volcanic black basalt

    Indonesia

    • Solid common opal (blue opal), usually associated with some opalised wood
    • Solid black opal

    Mexico

    • Solid fire opal (bright yellow, orange and red colours)
    • Solid water opal (bluish and green in colour)
    • Matrix fire opal
    • Solid purple opal, known as Morado opal

    Peru

    • Solid common opal (blue and pink opal), sometimes containing tiny translucent zones with play of colour

    United States

    • Solid common opal found in Oregon (light to pastel blue opal, and sometimes pink opal)

    Other countries that produced precious and fancy varieties of common opal include Czech Republic and Guatemala. Surprisingly, NASA (2008) has also found evidence of hydrated silica deposits (commonly known as opal) on Mars, which indicates that water bodies were once present in the planet.

     

    TYPES OF NATURAL OPAL

    Natural opals are opals that have not been synthesized, treated or enhanced in any way, apart from cutting and polishing the opal to reveal its colours. There are three main types of natural opal: solid, boulder or matrix. The following type of opals can have different varieties depending on its two main characteristics which are body tone and transparency.

    Natural Opal Type 1 (Solid):Solid opal is presented in one piece and in its natural state (apart from cutting and polishing) with a substantially homogeneous chemical composition. In simpler terms, this type of opal has a similar and uniform composition throughout it which allows the “play of colour” to appear.

    Natural Opal Type 2 (Boulder):This type of opal is presented in one piece and is commonly known as boulder opal. This is due to its structure that is naturally fused to the host rock in which it was formed, hence giving it a boulder-like appearance. Such boulder-like appearance is seen due to the heterogenous chemical composition of the opal and its host rock.

    Natural Opal Type 3 (Matrix):This type of opal is presented in one piece and is commonly known as matrix opal. This type of opal is ingrained between the crevices and holes of the host rock as infillings.

     

    MAIN VARIETIES OF AUSTRALIAN NATURAL OPAL

    Black / Dark Opal

    The most valuable and highly sought out opals are “Black opal”, where majority of it is only found in Lightning Ridge, New South Wales. This is because black opals have a black body tone that allows the “play of colour” on the opals to become visible more prominently than any other body tone. On the other hand, dark opal has a dark body tone that is not “pitch black” in colour.

    Light Opal

    Light opal is a type of opal with light body tone. Their body tone range between cream to white, which allows the soft, pastel colour on the face of the opal to appear. Light opals are the most common body tone for precious opal, where majority of it is produced in Coober Pedy (South Australia).

    Crystal Opal

    Crystal opal is a term used to describe an opal that is transparent or semi-transparent. This means light waves can pass through the opal allowing a vivid play of colour to appear on the face of the opal. Hence, the term “crystal” describes the degree of transparency on the opal rather than having a “crystalline structure”. Most crystal opals tend to have a light body tone, however there are some instances that crystal opals can have a dark to black body tones. Such dark to black variations of crystal opal are referred to as “Black crystals”.

     

    QUICK GUIDE TO VARIETIES OF OPAL

    Quick guide to black / dark opals: Dark grey to pitch black background (body tone) allowing the play of colour to become obvious – translucent to opaque.
    Quick guide to light opals: Light to white body tone allowing the play of colour to appear pastel in quality – Translucent to semi-translucent.
    Quick guide to crystal opal: Light to dark body tone with a clear background that allows a vivid play of colour to appear – Transparent to semi-transparent.

     

     

    OPAL CLASSIFICATION

    According to the Opal Association (2020), opals are classified based on their body tone (background colour) and transparency.

    Transparency ranges from transparent to semi-transparent, translucent to semi-translucent, and opaque.

    Body tone is classified as black opal, dark or semi-black opal, and light opal.

     

     

     

    BASE BODY TONE GUIDE

    According to the Opal Association (2020), the base body tone refers to the darkness of the opal ignoring the play of colour and brightness of the stone. The base colour can only be determined by looking down on top of the stone and ignoring the material on the back of the stone. Only those stones ranking N1 to N4 are considered black opals – attracting additional value associated of this class.

    Whereas lower-level ranking stones are classified as either dark opals (N5 & N6) or light opals (N7 to N9).

     

                       Figure source: Opal Association

     

    CURRENT MINING LOCATIONS IN AUSTRALIA

     
    Figure Source: Gemological Institute of America (GIA)

    According to Geoscience Australia (2020), majority of opal deposits are distributed along and within the Great Artesian Basin (GAB) region (see map). As a result, opal mining has become a way of life for many miners from small outback towns such as Lightning Ridge, Opalton, Coober Pedy and White Cliffs. Miners often follow structural features such as fractures and faults to look for potential areas of opal exploration. In addition, these small rural mining towns economies are also substantially dependant on opal-related tourism and fossicking.

    Geoscience Australia (2020) suggests that the first opal was mined commercially at Listowel Downs, Queensland in 1875, and later in White Cliffs, New South Wales in 1890. The discovery of opal later expanded towards Opalton, Queensland in 1896 and was followed by the accidental discovery of Lightning Ridge, New South Wales as an opal rich location in 1905 by a couple kangaroo shooters.

    Today, Coober Pedy is the main producer of light opals. Other mining locations in South Australia includes Andamooka (limited mining activities) and Mintabe (limited mining activities). On the other hand, Lightning Ridge is the world-renowned producer of high-quality black opals.

     

    Outlined below are other mining locations in Australia and the type of opals that can be found within these respective regions.

    Lightning Ridge region

    Lightning Ridge is the largest producer of sensational, fine-quality black opal. Despite a recent decline in production, Lightning Ridge is still the largest producer of black opal in the world (National Opal Collection, n.d.). These stunning black opals can be found in nodules, which are commonly known as “nobbies”. Other variation of opals such as white and crystal opal can also be found in the region. In addition, opalised fossils of various plants and animals have also been discovered within the area (Hsu, Lucas, & Pardieu, 2015). Outlying areas of Lightning Ridge such as Grawin, Glengarry, Sheep yards and Mulga Rush has also uncovered thin seams of opal. According to Hsu, Lucas, and Pardieu (2015), underground mining is more prevalent within this region due to the restrictions enforced by the NSW state government on open cut mining.  

    Queensland Fields

    The Queensland fields is an opal rich region where more than half a dozen fields are dedicated to opal exploration (see map). The Queensland fields consists of opal mining locations such as Yowah, Quilpie, Jundah, Opalton and Winton. These opal mining locations mainly produce quality boulder opals which are extracted from desert sandstone formation. The boulder opals found in these mining locations produces almost all the world’s boulder opal. According to Hsu, Lucas, & Pardieu (2015), underground mining and open cut mining is present within these regions, however the latter is more prevalent. Unlike nobbies, boulder opals can be found in siliceous ironstone concretions addressed as “lily pads” or “nuts” by local miners depending on how they are formed. In addition, the potential boulder opal concretions are all individually split by hand to uncover its contents; only a limited number of concretions yield opals. The process of producing boulder opal is contrary to the washing process practise in Lightning Ridge.

    Coober Pedy

    Majority of the world’s light seam opal is produced at Coober Pedy. The town reached its highest peak during the 1970s when there was a high demand for opal and copious amount of it was mined (National Opal Collection, n.d.). However, Coober Pedy’s opal production has fallen significantly from its highest production.

    White Cliffs

    Opal mining in White Cliffs began in 1880s where majority of the opal produced were fine quality light seam opal material (National Opal Collection, n.d.). A couple of decades later, the town developed and produced large volumes of opal. However, around 1930s the mines were some what depleted. Today, the mining field continues to hold hope and potential for future finds.

    Andamooka

    Andamooka is famous for producing crystal seam opal and was discovered around 1935. However, the field was depleted and there was very little commercial production that exist since the 1970s. Today some opal mining continues but, majority of the remaining local population is employed in the nearby uranium mines in Roxby Downs (National Opal Collection, n.d.).

    Mintabie

    Mintabie was first discovered as an opal-rich region in 1978 where a group of people found fine quality light and black seam opal. Within 10 years since the first opal discovery, the mines within this region has been depleted by the enthusiastic local miners. Now, limited mining activity exist in the area (National Opal Collection, n.d.).

    Lambina

    Previously, the area of Lambina was used as farming land. However, during the first discovery of opal in 2002 in comparatively shallow levels, around 300 miners flocked the area and arranged to use the field for opal exploration. Although there were some exciting finds, this field was comparatively small in production level (National Opal Collection, n.d.).

     

    FORMS OF OPAL

    Nobby

    Nodule or more commonly known as “Nobby” are spherical or sometimes cone like in shape (known as a witch’s hat), which are mostly found in the outlying areas of Lightning Ridge. Nobbys can also form as random blobs. A nobby is a natural raw form of opal that is directly extracted from the underground mines prior to being cut and polished. The three main features of a typical nobby is its crockery top (white top material), colour bar (which sits just under the crockery top layer that contains the colour of the opal) and a potch bottom layer.

     

    Seam

    As the name suggests, seam opal is flat and is formed by silica rich water running through the cracks and the seams on the earth. Most of the time, the opal seam is found horizontally in between the layers of rock. However, vertical opal seams can also be found. Sometimes these seams can run continuously over an extensive area forming large sheets or slabs, but more often they stop and start.

     

    Opalised Fossils

    Opalised fossils are simply the amorphous replacement of ancient remains of a plant or animal that has been trapped in sedimentary host rock. Majority of opalised fossils found within the Lightning Ridge region are opalised land and water creatures such as dinosaurs and freshwater animals.

     

     

    References

    Geoscience Australia (2020). Opal.Retrieved from https://www.ga.gov.au/education/classroom-resources/minerals-energy/australian-mineral-facts/opal

    Hsu, T., Lucas, A., & Pardieu, V. (2015). Splendor in the outback: A visit to Australia’s opal fields. Gemological Institute of America[GIA]. Retrieved from https://www.gia.edu/gems-gemology/winter-2015-splendor-outback-australia-opal-fields#item-4

    NASA (2008, October 28). NASA Orbiter reveals details of a wetter Mars. Retrieved from https://www.nasa.gov/mission_pages/MRO/news/mro-20081028.html

    National Opal Collection (n.d.). The Australian Opal Fields.Retrieved from https://www.nationalopal.com/opals/australian-opal-fields.html

    Opal Association (2020). Opal Industry Nomenclature.Retrieved from https://opal.asn.au/opal-information/opal-industry-nomenclature/