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What is an Opal?

Opal:

Si2.nH2O Non- Crystalline form of Hydrated Silica

Hardness:

5.0 - 6.5

Cleavage:

None. Conchoidal fracture (Boulder opal and Yowah nut opal has good cleavage as it splits in mirror image).

Colour:

Various White to Black base or background colours and vast array in play of colours

Specific Gravity:

Very low 1.9 to 2.3

Habit:

Massive

Lustre:

Opalescent, Vitreous to resinous

Occurrence:

As Veins and Nodules (nobbies)

Streak:

White

Uses:

Ornamental or Cut into gems for jewellery

Australia produces over 90% of the world's supply of precious opal and 100% of the highest quality gems.

Opal contains 4% to 9% water and the precious opal (only representing a small portion of the opal mined) consists of close-packed array of regular silica spheres. The size of the spheres is less than 1/1000mm. Opal, derives its name from the Greek word, "Opalus," meaning 'to see a change (of colour)'.

Description: Description: Description: [ image ]In precious opal the arrangement of spheres is in orderly layers, and light passing through the spheres is diffracted at the void and layer interface to produce the vivid play of colour associated with opal. Larger silica spheres are associated with more sought after colours, such as red, the smaller are the green to violet. In the spectrum of gemstones that display special optical effects, few materials can rival the pre-eminence of opal for variety and beauty. This material exhibits a broad spectrum of colour that changes as impinging light plays off the regimented layers of sub-microscopic spheres that form the basic structure of opal. In the best examples, dramatic splashes of colour are juxtaposition like puzzle pieces producing a wide range of patterns. Although some combinations are spectacular, others are more subdued, but all are always interesting because of the exceptional range of possibilities.

Because of the curious nature of opal formation, it is possible for a variety of organic forms to be systematically replaced by opal bearing fluids that faithfully replicate the object during the replacement or fossilization process. Shell and other forms of plant life can be Opalised in a slow, methodical process, that reproduces the shape of the original life form in both "precious" or "common" opals.

The uniqueness of these opals, familiar to the ancient seascape of Australia, has been faithfully mimicked in a highly transparent opal exhibiting any combination of colours across the spectrum but predominantly a blue/green play of colour, like the sea it started with. The desirability of any example of opal replacement is dependent on a combination of factors including shape, form, colour, completeness and especially the brightness or quality of the actual gem material.

Australian opal has been dated in the late Cretaceous and early Tertiary periods. Description: Description: Description: [ image ]

It has hardness ranging between 5-6.5, is brittle with a conchoidal fracture and some light varieties fluoresce white or yellow under long and short wave ultraviolet light. This property is used on South Australian fields such as Coober Pedy to noodle opal from tailings, which are run on conveyor belts through darkened sheds, past UV lighting.

Black opal is generally more sought after and more valuable than light opal. In Australia the major opal producing fields for black opal are Lightning Ridge in New South Wales and Mintabie in South Australia.
Lightning Ridge opal is usually found as nobbies- small blocks, pillows, spheres or hat shaped stones ranging from around 1-5 cm across. The stones usually have light grey appearance when found due to a thin outer layer of grey potch. When stones are clipped they reveal black potch inside along with any colour bar.

Opal from Lightning Ridge is considered to be the best and brightest in the world.

Why, we might ask, is black opal black? The reason for blackness in volcanic opal is the presence of impurities of iron oxides, scattered like fine dust through the substance, in sufficient quantity to impart a jettiness of colour. Black opal from Lightning Ridge has carbon along the pseudocrystalline boundaries. The base colour of white opal is a property of the structural imperfections in the stacking arrangements of the basic silica microspheres that compose opal; these imperfections scatter and diffract white light. Black opal absorbs most of the white light, which impinges upon it, save for that fraction which is diffracted as glorious colours.

The debate on the origin of vein opal deposits occurring in the Cretaceous sandstones and claystones of the Great Australian Basin has recently polarized into two conflicting geological theories. These are the historical and long held Deep Weathering theory and the newer "Syntectonic" theory of opal formation.

Description: Description: Description: [ image ]The older "Deep Weathering " theory of opal genesis advocates a process of amorphous silica generation via the breakdown of feldspar to kaolinite, or smectite to kaolinite, in a very high rainfall climatic environment. This amorphous silica is said to have then precipitated very slowly from downward moving ground waters in cracks formed by shrinkage and desiccation at permeability barriers. These ideas have given rise to the concept that the vein opal deposits of the Great Australian Basin are "Sedimentary" (Hence the term Sedimentary Opal). In this genetic model faults and fractures are only considered to have acted as a static passive conduit for the percolation of the silica-laden ground waters to depth through the rock mass. The "Deep Weathering" model advocates no direct genetic relationship between faults and opal veins.

The new "Syntectonic" theory of opal vein formation proposes a far more dynamic process for the genesis of opal veins enclosed within Cretaceous sedimentary rocks of the Great Australian Basin. This model advocates a process involving the generation of fault controlled cyclic, fluid pressurized systems which formed opal vein arrays linked to nearby faults. In this genetic model the opal is considered to have been deposited actively and rapidly in fault generated hydraulic extension fractures, by fluids supersaturated with respect to amorphous silica, and at temperatures Faults and associated Breccia pipes are well above those typically involved in the surface processes (i.e. <100°C). considered to have formed by Description: Description: Description: [ image ]tectonic processes coincident with opal vein formation and to have acted as pathways for silica-laden fluids, which moved through the rock mass under hydraulic pressure. As regional and local scale tectonic processes deformed the sandstones and claystones in selected areas, fluids were squeezed and pumped through the rock mass along faults and fractures, depositing opal and forming veins at sites of lower pressure, in fissures opened by the faulting. Thus the "Syntectonic" model advocates a direct genetic relationship between faults and vein opal deposits in the near surface sedimentary rocks of the Great Australian Basin.

 


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