What is an
Non- Crystalline form of Hydrated Silica
5.0 - 6.5
None. Conchoidal fracture (Boulder opal and Yowah nut opal has good cleavage as it splits in
White to Black base or background colours and vast array in play of
Very low 1.9
Vitreous to resinous
As Veins and
or Cut into gems for jewellery
produces over 90% of the world's supply of precious opal and 100% of the
highest quality gems.
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)'.
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
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.
opal has been dated in the late Cretaceous and early Tertiary periods.
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
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.
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.
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 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