Boulder Opal

Boulder Opal is an amazing and beautiful stone. From all the hundreds of gems created by nature, no other gemstone beats an opal when in its natural form (before cutting).

What is Opal and how was it formed.

Opal is Hydrated Silica. Hydrated means that the silica contains water. Silica is also known as silicon dioxide (silicon is one of the most abundant mineral in the Earth's crust). Silicon is a non-metal chemical element that has semiconducting properties. Dioxide tells us that the silicon has two oxygen atoms as part of its make-up.

Structure

During the Tertiary period (65 million to 2.6 million years ago) in the Australian outback, rocks which now contain opal were subject to significant weathering. Water seeped through sandstone, leaching silica particles and filtering out contaminants until it became trapped by underlying layers of impermeable stone. Over time some of the water dissipated leaving a high concentration of silica particles.

Silica has the natural tendency to try and reduce its contact with water. If the conditions were conducive and stable, these small silica particles eventually merged together to create spheres, forming, with the water, what is known as a 'sol'. One important factor for the play-of-colour to exist is that the spheres must grow to a size no smaller than 200 and no larger than 350 nanometres (a nanometre [nm] is one-billionth of a metre), which is exactly half the wavelength size of colour—400-700 nanometres.

In time these spheres attract one another but do not merge, and when enough have come together they sink to the bottom of the solution. These spheres are negatively charged and give a strong inter-particle repulsion; because of this repulsion the particles glide around one another after sedimentation (rather like lubricated ball-bearings in a bucket). The viscosity of the sol determined what size the spheres would be before settling out of solution. Hence spheres of similar size tended to settle together.

For there to be colour in opal the silica spheres need to be the same size and grouped together in an orderly three-dimensional array. Because of this uniformity in size, the spheres form tiny regular sized pits which are able to affect the wavelength of light.

Opal starts out as a liquid then turns into a gel and then eventually hardens. For this process to occur,enough water needs to dissipate so that the structure will stay in place. The water content in opal varies between 2% and 20% (it is usually 6 to 10% in precious opal).

Pattern Stacking faults in the silica sphere structure of opals produce many distinct colour patches and determine the pattern of an opal's play-of-colour.

How the silica sphere structure affects the wavelength of light to create colour.

In opal, both diffraction and interference plays a role in the play-of-colour.

Diffraction in opal is the result of light hitting the gaps formed by the spheres (grating) and then being split up into its spectral components. The grating pattern changes the wave length of the reflected incident light, which results in the different colours (see picture below).

How the silica sphere structure affects the intensity of the colour...

Interference occurs when part of the light gets reflected from the surface of the silica sphere while another part gets refracted inside the sphere, and then reflected out—this increases the intensity of colour and brightness. It is also interesting to note that the silica spheres have a series of layers simular to an onion. These layers are believed to contribute and multiply the interference effect (see picture below).

Opal can be any colour; the size of the many gaps or pits determines which colour is seen. In precious opal, the larger spheres create a red colour, which can change depending on the viewing angle*. It is also the rarest colour as it takes longer for the spheres to grow to that size.

Here is the order of colour and corresponding sphere size (smallest-largest): violet - 200 nm, indigo - 222.5 nm, blue - 237.5 nm, green - 255 nm, yellow - 285 nm, orange - 295 nm and red - 325 nm.

*There is a rule in physics called ‘Bragg’s Law’ which explains why opals with a red play-of-colour are also usually able to show all other prismatic colours too when the stone is tilted and viewed from lower angles.

Only a small percentage of Opal which is mined, is considered of high enough quality to be set into jewelllery, Color Flash or Opal Fire are two terms that are used to describe the magical colour of Opal. The quality of color can mean the difference of tens of thousand of dollars.

Just as important to the price of Opal is the pattern on the stone, some patterns are so rare that they can easily double the price of the stone.

In Boulder Opal every polished stone will have its own unique identity due to its pattern.

A famous opal miner expression is “opal is where you find it”. It seems no matter if you’re a Geologist, an experienced opal miner, or relying on a trusty divining rod.

Finding thousands of boulders does not mean you will find opal in any. If you are lucky and open a boulder to discover fine opal gem, then appreciate and respect it. For it might be many, many years before you find some more.

So why is it that out of all gemstones the Opal was the stone that had its name derived from the word 'precious gem'? Not diamond, ruby, emerald of sapphire but the Opal. The ancients who decided such things would not be clouded by commercial propaganda but to put it simply... telling-it-like-it-is. Giving the Opal stone the one and only title of 'precious stone'.

Click here to view our Opal jewellery

Mining Boulder Opal

Our opal mine is at an extremely remote location called Vergemont cattle station. Travelling to ur mine from Brisbane takes a total of 35 hours, this includes 3 hours on a rugged bush track.

The picture below shows our mine on google maps (you can zoom out on the picture below to see how remote it really is).

View Larger Map

Below is a picture of our mine, after all mining has been completed we will return the environment back the way it was. Most opal mines hae a small camp, a mine can be cut off from the nearest town for weeks at a time due to floods.

To locate the opal, the 1st step is to drill into the ground to find evidence of opal. Once opal traces have been discovered the miner drills the earth looking for the opal level (stone layer that has a high possibility of containing opal).

Once the opal level has been found, the next step is to try and locate a fault in the level - as there is a tendency for opal to be found close to faults and disturbed ground. The miner will drill a series of holes looking for a drop in the level depth. If a drop is found he will continue to drill holes, narrowing the distance between the holes until the fault is discovered.

20 Tonne excavators are used in most Boulder Opal mines as large quantaties of earth need to be moved to find the opal - it's a fast way of getting down to the opal level. The biggest challenge for most opal miners is the cost of diesel and maintenance. All Australian opal is becoming significantly more expensive to mine.

The picture below shows different levels and a fault line that opal miners are looking for.

A bullbozer works the ground while other miners crack the unearthed rock using large hammers - they are looking for the opal inside. Most Queensland opal forms inside the rock, hence the name Boulder Opal.

Discover the jewellery that has been created from all this hard work - click here

	From Lizzie, in Central Victoria My pendant is beautiful and fantastic value for money. Its very well made and we are impressed by the quality. I was thrilled to acquire it because I already have a round garnet pendant of yours & I have had just so many compliments on that one. Thanks for being you! Do I ever need to go into a jewellers in my local city? Probably not! LIZZIE, Australia (11-11-2011)

	Great quality item and just as described. Super fast shipping and professional service. Pleasure to deal with. Neil, Australia (07-11-2011)

	the person for whom I purchased this has not yet received it and not being an expert on opals I cannot comment on good the item is. This much I can say, I am most impressed with the customer service of JGJJewellers. Thanks to everyone involved and especcially to Geoff for being so prompt when answeing my enquiries. Ian, Australia (30-10-2011)

Boulder Opal

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