An interesting specimen of boulder Ironstone matrix opal

A recent specimen of boulder ironstone matrix opal which has come across the research table of the opal academy. The field of origin knowledge of the specimen is unknown at this time. The specimen is a part of a collection of opal specimens from from our colleague Peter Sherman, and was given to the academy because of its gemmological interest. This opal component of this gemstones shows characteristics of hydrophane opal.

hydrophane type ironstone boulder matrix opal.

A broken oval cabachon of hydrophane type boulder ironstone matrix opal. The top section of the cabochon has been immersed in water and the bottom section of the stone shows the normal state.

A photomicrograph of a portion of the ironstone boulder matrix hydrophane opal partially showing the difference between where the opal absorbed water (darker portion at top), and the dry white portion below.

The ironstone matrix of this opal is typical of boulder ironstone matrix opal from Queensland opal fields, however the precise field location is unknown. Previously we had heard of a similar possible occurrence of boulder matrix that may have been hydrophane type opal, scant details were given in a discussion at an opal symposium in Winton, Queensland. At the time and after a request we received some similar material for examination from Cody Opals in Melbourne, however on examination it was difficult to determine the true nature of the Cody samples. This sample in the pictures above however was significantly more absorbent and worthy of sharing with the gemmological community.

opal which may separate from its ironsone host

In some instances after cutting and polishing, the opal component of the boulder opal can separate from the ironstone. There may be a sandy or silt layer found between the opal and the ironstone rock which interferes with the adhesion between the two components. A test of this can be used by immersion in water (or another liquid). This is illustrated in the photographs below. The water will “whet” the surface of both the opal and the ironstone, and hence make the opal layer more visible. It is this scenario that makes it difficult to determine if the specimen is hydrophane, or if it is just the water being included between the opal and the ironstone.

The two pictures to the right show a subtle defect that may occur in a very small amount of boulder opal where the opal layer that is attached to the ironstone may partially separate. The photo on the left is a photo that shows this separation. The photo on the right is a photo of the same opal that has been immersed in water, note that the water has “whetted” the contact between the opal and the ironstone. Note that the specimen is not an indication of the opal being hydrophane.

A specimen of boulder opal where the opal is separating from the ironstone

The same specimen which has been “whetted” after immersion in water.

a gemmological point of interest

the indication that the opal layer may be separating from the ironstone leads us to look at the possibility of a treatment for this variety of ironstone boulder opal. it is possible and has been noted in gemmological treatment literature that it is possible to fill the join between the opal and the ironstone with a suitable resin that will permanantly seal the opal back onto the ironstone base. This becomes a difficult treatment to both analyse and detect. We will discuss this more in our posts on opal treatments.

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An interesting specimen of boulder Ironstone matrix opal

Opal Academy examines an interesting specimen of ironstone boulder matrix opal with characteristics of hydrophane..

Magnetically attracted boulder ironstone matrix opal

Opal Academy examines a specimen of magnetic ironstone boulder matrix opal.

What is Black Opal?

Precious black opal is an exquisite gemstone, opal is often referred to as “the Queen of Gems”

The Story of Hydrophane Opal

Precious hydrophane opal is a variety of precious opal (opal that shows a play-of-colour) that is absorbent. Typically it absorbs water, however the porosity of the material also is indicative of the material being susceptible to absorbing other colouring agents. As such it poses a difficulty for gemmologists in identification procedures.

What is Boulder Opal?

Boulder opal is in most instances the name given to opal that occurs in ironstone concretions of varying sizes. When cut and polished with “the rock” is incorporated in the gemstone. In a nomenclature discussion this is determined to be defined as “Opal and rock”. This is the description given in the opal nomenclature which we continue to refer to in other blogs, and descriptions on the Opal Academy website.

What is matrix opal?

Matrix opal has been defined as “opal presented in one piece where the opal is intimately diffused as infillings of pores or holes between grains of the host rock in which it is formed”.

In the original nomenclature published in the Australian Gemmologist Volume 19, Number 12, 1997 which you can read in the literature section of the website, or you can check out the Nomenclature page here………

A discussion on Imitation Opals

Earlier this year and due in part to not being able to meet as everyone will say “covid19” I was approached by the Scottish Gemmological Association to give a presentation via “Zoom’ to the members for their usual annual conference. This was something significant and I was really privileged to do. The presentation , albeit with one or two “quirks” is available here for anyone to view. The presentation runs for about 40 minutes.

What is Gemmology?

The study of opal gemmology follows the normal procedures of non-destructive gemmological testing. These procedures examine the observable qualities of gemstones which include the physical or mineralogical properties of opal and more particularly measurable observations of refractive index (RI), Specific Gravity (SG), examination under exposure of excitation Ultraviolet light, and Microscopic examinations of opal characteristics such as imperfections and inclusions. Advanced material science methods of examinations for gemmological purposes is also beginning to become a necessity for the modern gemologist

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.

Synthetic and Imitation Opals

The first attempt at manufacturing synthetic opal was made soon after the determination of the structure of opal in 1964 Sanders. Details of the procedure used to first synthesise opal are preserved in the original patent documents registered by the CSIRO in Australia, Great Britain and the USA during 1964. The original patents for the synthesis procedure were finally accepted some seven years later, on October 11th 1971.

Triplet Opals

Triplet opals then are an artificial opal that is constructed from three pieces, in someways a triplet opal is just a doublet opal that has had another piece of material adhered to the top. So to create a Triplet opal all you need to do is adhere a clear or transparent material to over the top of the “opal” layer! As with the black material used as the base of a doublet opal, the clear layer over the top can be any material that is clear and transparent. Again it may be glass, plastic or anything else.

Doublet Opals

Doublet opals come in two basic forms, a doublet of two pieces to imitate a black opal, or a doublet of two pieces to imitate a boulder opal. In the case of a doublet imitating “Black opal” the doublet usually consists of a thin laminate of “opal” glued or adhered to a black substrate of some material (the substrate can be be anything that is black, a few examples may be black potch opal, or black glass, or something painted black), anything black will do!

Composite Opals

Early Synthetic Opal

Soon after the ordered, sphere-based structure of opal was revealed by CSIRO scientists, innovative potential synthesisers of this material started to postulate the various methods by which synthetic opals could be manufactured. All that was needed were suitably sized spherical particles of a suitable transparent medium. In fact, any transparent material capable of being flocculated and deposited into a three-dimensional ordered spherical array would be suitable for producing imitation opal; provided the spheres produced were of suitable size and optical transparency, and that they were arrayed in the appropriate Bragg relationship.

Treatments

Equipment & Procedures

Chip Opals

Chip opals are a little different in their construction, usually the opal component (again either natural opal or imitation opal) is encapsulated into a some form of envelope. The opal chips are mixed together, usually with a resin based material and then contained in some sort of capsule. The chips can be tightly packed, or just loosely packed depending on the type and effect to be obtained.

Chip opals are found to imitate all three of the natural opal varieties, light opal, Black opal and boulder opal.

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GemmologyAnthony SmallwoodAugust 13, 2026