opalwhatisit

OPAL, what is it? (see also "Mining for Opal)

Opal is an amorphous form of dehydrated silica and a common occurrence throughout the world, but only as common opal and seldom as precious opal.

It was formed approximately 70 million years ago in the tertiary period. About 135 million years in the past, the ground where it is found today was close to the bottom of the sea. Therefore sea creatures like shells and belemnites can be found in opalised form. The silica which opal consists of, was dissolved and transported by groundwater and filled up the hollow spaces in the ground. In these spaces it hardened and formed opal.

Characteristics of opal:

Chemical formula: SiO₂ n H₂O (Siliziumdioxid and water)

Hardness: 5.5 - 6.5 (Moh’s scale)

Water content: 3.5 - 12%

Specific gravity: 2.1

Refractive index: approx. 1.45

When silicon is mixed with oxygen, it becomes silica, a common mineral. Mixed again with water it turns into silicon dioxide. In crystal form it can be quartz, in cryptocristalline form we have agate and in amorphous form (that is without crystal form) and mixed with water we have opal. Because the water content can go up to about 12% (by weight of stone), there is the danger that the opal dries out and cracks. The lower the water content, the more stable is the opal. The hardness of opal is about the same like glass, around 6 on the scale, where diamond is 10 (the hardest).

An approximately analysis reveals following trace elements in Coober Pedy Opal:

6.1% H₂O	0.4% Na₂O	0.01% ZrO₂
1.8% AL₂O₃	0.2% Fe₂O₃	0.05% MgO
0.8% CaO	0.01% TiO₂	0.002% Ag₂O

Source: Len Cram, Beautiful Australian Opal, A field guide, RBA, Australia 1995, ISBN 1862730504

Where do the colours come from?

Opal consists of an array of well packed silica spheres in a three dimensional grating. When this array occurs in a certain regularity, white light is split up into colours. This diffraction is caused by the voids between the spheres. The size of the voids is controlled by the diameter of the spheres. For example the colour red appears when the diameter is about 3500 Angstroms, and blue is the result of a diameter of 1500 Angstroms. That means bigger spheres will have a wider angle in between and are able to split up light with a lower frequency, e.g. the colour red. But all other angles are also possible so we have the whole spectrum. Smaller spheres produce only green/blue and very small ones only blue. And for a good play of colours, the angle at which the light enters the stone and is diffracted is critical. Faults in growth or stacking of silica spheres create grain boundaries where the colour abruptly changes.

Potch, that is Opal without any colours, had either the void between the spheres filled in with silica cement, the array is too irregular, or it has too many impurities. Also, the spheres could be too big, too small or just have different sizes.

Some History about Opal

Opal was known in ancient times and probably first found in India. It was introduced into Greece during the first century B.C. It was called opallios, this name changed slightly over the centuries to opal, what literally means ‘to see a change (of colour)’. The Romans valued opal above all other gems and it came probably from open cut mines in Hungary.

It was also known in several countries like Indonesia, Java, Mexico and Turkey. But the Greeks and Romans obtained their opal probably from Anatolia (Turkey) or India, as mentioned before. Much later, opal was mined underground in Hungary, in a town named Opalbanya. But this opal had a high water content and had to be brought up to the surface in stages over a period of time. It cracked very easily. The opal from these times would be considered as rather low quality compared to today’s standards.

Until 1829, opals were considered lucky. In this year Sir Walter Scott published his novel Anne of Geierstein, in which an opal reflects the changing fortunes of the heroine. Unfortunately, some people got the idea that opal might have properties of evil influence. The result was absurd, twelve months after the publication opal was down to half of its former value, and falling rapidly out of fashion. This was the beginning of a superstition which in some countries is still present.

Dates and facts:

· 1851, opal was found in Mexico, near Queretaro. But it was twenty years later when serious mining commenced.

· 1841, opal was found in South Australia, near Angaston by Johann Menge, a German prospector.

· 1870, nodular pieces of opal were found in basalt in Gippsland (Sunbury and Gelantipy).

· 1871, opal was found in Queensland, Australia. (Listowel Downs and Springsure).

· 1915, opal was found in Coober Pedy, then called the ‘Stuart Range Opal Field’.

The biggest known opal ever found up to now (in 1995) is called Jupiter 5 with a weight of 5.2 kilograms! It was found in Coober Pedy, South Australia in 1990, and was sold for 220’000.-A$.

Today, over 90% of all opal comes from Australia and it is estimated that up to now less than 10% of the opal was found. According to a government estimation there should be opal for more than 10 trillion dollars still in the ground. And the possible opal bearing area is more than 400’000 square kilometers.

But opal is also mined in Mexico, the United States, South America and Indonesia. This is mainly volcanic opal, while Australian opal fields are mostly sedimentary deposits. Although volcanic opal is of good quality, it tends to crack due to the high water content and it will never compete against the Australian fields in regard of production cost and volume, unless new sedimentary deposits are discovered.

History about Australian Mining Areas

90 % of all opal comes from Australia. Therefore it is of some interest to know the main opal producing fields and some history about important men who discovered opal and developed an industry.

The first recorded mining and sale of opal comes from Springsure in Queensland in 1872. It was discovered by Mr O’Brien, a store employee, and held afterward under lease by the local telegraph officer Mr T.F. Batho. This opal was of volcanic origin, appeared only in bean-sized pieces and was difficult to mine. They did not attract much attention, as other finds were discovered in 1873.

In 1875 Herbert Bond registered a company in Pall Mall, London. The company held twelve claims in Queensland. In 1877, the Grey Range became the scene of a sudden rush of opal-fever. A coach line had 6000 horses on the roads to supply fast developing town sites west of Charleville. Towns like Blackwater, Quilpie and Kyabra were founded in this time. By the middle of 1878 the storekeepers had arrived, but about this time Herbert Bonds Company collapsed and spelled an end to the miners market.

The real founder of the Australian opal industry is Tully Cornthwaite Wollaston. In 1888, with 25 years of age and after organising a syndicate to back him financially, he set out from Adelaide to central Queensland. He traversed 1100 kilometres in seven weeks, during the worst drought then recorded. He met a miner in Kyabra, Joe Bridle, and bought 61 stones of top quality stones from him. When Wollaston went to London, he first had difficulties to sell the stones, they were of unknown quality. But the business firm Haslock Bros was ready to give this new opal a chance, and the first parcel of cut Australian opal was ready for the American market They sold with a high profit, but when Wollaston went back to Australia the syndicate was not prepared to explore the leases he had taken out. So he withdrew from the syndicate and went out in the fields again.

On the Stoney Creek field he bought 60 pieces of red grained opal in the size of walnuts of Bill Johnson, working the “little Wonder” mine. They were the most beautiful opal he had ever seen, and the price was merely one thousand pound. From then on, Wollaston gradually took to buying and selling parcels of rough opal. Opal trading soon became brisk and prices soared upwards.

In 1903, Charlie Nettleton, a miner from Lightning Ridge (then Wallungulla), tried to sell black opal but it was declared valueless. But Wollaston saw the potential and bought the parcel. Soon this field attracted many miners and became famous for its black opal.

In 1915, a gold prospecting party camped near the Stuart Ranges in South Australia. Bill Hutchinson, the fifteen year old son of one of the prospectors, found opal floaters lying on the ground. Word got around and soon two miners from White Cliffs, Jim and Dick O’Neill, headed for the field. And they found 17,000.- pounds worth of opal! The government constructed watertanks and in 1925 the settlement was officially given the name of Coober Pedy, supposedly Aboriginal words meaning: “white man in a hole”.

Soon opal was discovered in other places in South Australia like Mintabie and Andamooka. In 1956 opal mining was at very low activity with about 30 men in Coober Pedy, 10 in Lightning Ridge and in Queensland it was at a complete standstill. Then mechanisation came, bulldozers and calweld drilling rigs came onto the fields. A new era of mining had begun!