Boulder Pipe Opal

Boulder Pipe Opal is found in the Central Queensland opal fields in Australia. As the name suggests it is opal formed in a pipe or tubular formation. These roundish tubes are formed in host rock, which is mostly sandstone. The sand stone potch enhances the natural fire color in this crystal opals. 

Most pipe opal is green to blue fire colours but some have sunset bright fire opal colors but this is rare. The tubular pipe opal can be hollow or tubular.

Boulder opal is the term used to describe gem-quality opal that occurs in veins and pockets in ironstone concretions, and retains some portion of the ironstone when cut. Most boulder opal on the market today is found in association with the Winton formation, a broad belt of Cretaceous sedimentary rock that covers the entire center of the state of Queensland, Australia.

Boulder Pipe Opal
Boulder Pipe Opal.
Photo and collection: Flashfire-Opals

After several years of intense mining in the late 19th century, interest in Queensland boulder opal waned as the harsh climate drove miners to more lucrative deposits in areas such as Lightning Ridge and Coober Pedy.

In Winton, the opal is deposited in small veins or voids within the ironstone and is known as boulder matrix opal.

Mining for pipe opal has not been regular with small pockets of pipe opal being discovered. It can be several years or decades before more pipe opal is found in large quantities.

Be careful not to confuse belemnite opals with pipe opal.  Belemnite opal is also tubular and solid or hollow like pipe opal but it is a fossil from South Australian opal fields.

The famous tubular shape Virgin Rainbow is also not a belemnite but formed under the pocket left by a dead belemnite. Pipe opal from the Queensland boulder fields is not a fossil.

Boulder Pipe Opal
The Virgin Rainbow opal
Photo: Richard Lyons courtesy South Australian Museum

Opal is sdicon &oxide with water, with the general formula SiO₂·nH₂O. Silica actually represents 85%-90% of the composition of opal. Scanning electron microscope studies of gem opal have shown that the phenomenon called play-of-color is the result of a regular arrangement of silica spheres that fonn a sort of honeycomb pattern, with uniform gaps between the spheres. These gaps create a three-dimensional diffraction grating.

The essential preconditions for the forination of these grids are:
(1) a clean silica solution, (2) an undisturbed cavity in which the solution can accumulate, and (3) time for water to evaporate and the spheres to line up at the bottom of a cavity.

See also:
Opalized Belemnite Fossil
Types of Opal With Photos

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