Lightning Ridge, Finch Co., New South Wales, Australiai

Regional Level Types
Lightning Ridge	Town
Finch Co.	County
New South Wales	State
Australia	Country

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Small outback town in Walgett Shire. The Lightning Ridge area is a centre of the mining of black opal and other opal varieties.

Located approximately 46 miles (ca. 74 km) north of Walgett.
Located in the parish of Wallangulla.

NOTES:

- Lightning Ridge is NOT located in the New England region.

- Boulder opal (i.e., opal in ironstone matrix) is sometimes incorrectly stated as being from the Lightning Ridge area. It is only found in the Queensland opal fields.

Mining of black opal commenced in 1901. Underground and open-cut mining techniques are used to extract opal from depths ranging from near the surface up to 25 metres underground. The host rock is the Cretaceous Griman Creek Formation, which is predominantly composed of freshwater (rarely estuarine?) claystones and sandstones.

In the immediate vicinity of Lightning Ridge, most gem opals are found as "nobby" opals. Nobby opal occurs as spheroidal or disc-like pieces of opal that range from about 1 to 15 cm in diameter, sporadically distributed through claystone layers.

Opalised fossils also occur sporadically. Fossils include bivalve shells, gastropod shells, dinosaur bones & teeth, turtle shell & bones, crustacean gastroliths, plant stems & fruiting bodies, plesiosaur bones & teeth, monotreme bones, etc.

At Lightning Ridge, opal occurs in a seemingly endless variety of topologies. The shapes and textures apparently formed via a range of processes, none of which is well understood. Topologies include complex septarian patterns in very fine and intricate honeycomb networks in melikaria and rare counter-septarian structures in shell-like laminae and husks, kernels and meniscus forms ("pools," "cups," and "dishes"), webs and "strings", spots, spheroids, and botryoidal blobs.

The mechanism of septarian formation has been described in detail; the term “melikaria” is used to describe features that have geometric patterns similar to septaria but lack a surrounding matrix.
Most of these structures represent delicate fractures that developed in silty clay as a result of dehydration. Very fine reticulations and “crinkled” textures commonly line cavities in plant fossils or in fissures between opalised structures and enclosing claystone. They are also seen sporadically on the outer surfaces of plant fragments, bivalve shells, and bones. These intricate boxworks have cavities that are empty, as in melikaria. However, our observations suggest that some may have developed when gelatinous silica hardened and shrank away from cavity walls during desiccation, a result of the interaction between the tensile properties of silica gel and fine, hydroplastic claystone minerals. Other septaria may be infills of syneresis cracks formed by the dewatering of unconsolidated sediment.

Commodity List

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Mineral List

Mineral list contains entries from the region specified including sub-localities

5 valid minerals.

Rock Types Recorded

Rock list contains entries from the region specified including sub-localities