Lightning Ridge Black Opal

Unique Lightning Ridge Black Opal
Unique Lightning Ridge Black Opal
Photo by Wood's Stoneworks and Photo Factory on Flickr

Lightning Ridge Black Opal it is like lightning bolts that flash and change as it moves. Lightning Ridge mine site in New South Wales, Australia.

Formula: SiO₂·nH₂O
Colour: Colorless, white, various
Hardness: 5½ - 6½

Lightning Ridge black opals are the most valuable in the world. Lightning Ridge is one of the few places in the world where the precious and highly prized black opal is found. Unlike other opal, the black opal contains carbon and iron oxide trace elements, producing a very dark stone which has hints of blue, green and red play of colour.

Lightning Ridge lies in a large geological feature called the Surat Basin, which is part of the vast Great Australian Basin. The Great Australian Basin covers 1.7 million square kilometres of eastern Australia. It was formed during the Jurassic to Cretaceous, when dinosaurs walked the Earth. About 140 million years ago, the basin contained a large inland sea which accumulated sediments that later hosted the formation of precious opal. The sedimentary host rocks are essentially horizontal because they were deposited on the seafloor and haven't been deformed much since. The rocks which host the opal at Lightning Ridge were deposited in shallow water near the edge of the basin, probably in an estuary.

Overlying the Cretaceous sedimentary rocks are sandstones and conglomerates that were deposited by streams and rivers in the Tertiary period, about 15 to 5 million years ago. Many of these younger rocks have hardened due to weathering processes to form silcrete and are commonly quarried for road materials.

Host rocks contained a variety of voids formed by the weathering process, which leached carbonate from boulders, nodules, fossils, cracks, hollow centres of ironstone nodules and horizontal seams. Most opaline silica deposited is common opal (or potch). It does not show a play of colour. Opal also fills pore space in sand-size sediments, cementing the grains to form deposits known as matrix or opalised sandstone.