Turquoise from Ammaroo, in Australia's Northern Territory, is a notable and rare find, with a history of commercial mining in the mid-20th century. The most significant historical period for this turquoise was in the early 1970s, although it was known to prospectors earlier. While a specific major discovery in 1952 is not widely documented, the area's history of turquoise exploration is well-established and offers insight into the unique characteristics of the gemstone found there.

Rarity and History

Turquoise is generally rare in Australia, and the Ammaroo deposit, specifically the Tosca Mine, is the only known commercial source of the gemstone in the country. The deposits are found in a layer of claystone within the broader mineral-rich landscape of Ammaroo Station. The rarity of Australian turquoise makes specimens from this location highly sought after by collectors.

Colours and Beauty

Unlike the intensely saturated "Persian Blue" turquoise, which is prized worldwide, Ammaroo turquoise is known for its softer, more muted tones. The colours range from a striking sky-blue to pale blue and even a subtle pastel blue. This unique coloration is a defining feature of the Ammaroo stone.

The beauty of Ammaroo turquoise is often enhanced by its "matrix"—the host rock remnants that appear as veins or patterns within the stone. These can be a beautiful dark brown or black, creating a distinctive "spiderweb" effect that adds character and individuality to each piece. Specimens are also found alongside other minerals like malachite and chrysocolla, which can introduce beautiful blue-green and green hues.

Quality and Challenges

While Ammaroo turquoise is valued for its unique appearance, much of the material found there is considered "carving" or "specimen" grade, with only a small amount of "gem" grade material suitable for high-end jewelry. The stone is often porous and contains a clay mineral called montmorillonite, which can cause it to absorb water and expand. For this reason, many specimens must be waxed to preserve their colour and integrity. This natural characteristic, while a challenge for lapidaries, adds to the gem's story and its reputation as a true piece of Australian geology.

A Deeper Look at Ammaroo Turquoise

The turquoise deposits at Ammaroo are a rare and unique part of Australia's mineral history, primarily centered on the Tosca Mine (also known as Tosca's Hope). Unlike many world-famous turquoise sources, the Ammaroo deposit is not found in volcanic or hydrothermal environments, but rather in a specific geological context.

Geological Formation

The turquoise at Ammaroo formed from low-temperature fluids within ancient sedimentary rocks. Specifically, it is found in layers of claystone and phosphatic mudstones of the Cambrian-era Arthur Creek Formation. This geology is unique and explains the distinctive characteristics of the stone.

The turquoise's formation is directly linked to the presence of copper and aluminum phosphates in the host rock. The "spiderweb" matrix often seen in Ammaroo turquoise is a result of the veins of the host rock—dark brown or black minerals like goethite and other iron oxides—running through the pale blue gemstone. The presence of other minerals like variscite, malachite, and chrysocolla also contributed to the varied colours of the gemstones found here.

Mining History and Quality

Commercial mining at the Tosca Mine was limited and primarily occurred in the early 1970s. The operations were small-scale, with a few pits bulldozed to expose the turquoise-bearing layers. Reports from that period indicate that while "carving" and "specimen" grade material was found in abundance, very little "gem" grade material was ever recovered. This is partly due to the stone's inherent porosity and a clay mineral called montmorillonite, which makes the stone prone to absorbing water and changing colour.

Indigenous Heritage and Modern Context

The Ammaroo area, and the Tosca Mine in particular, holds significant cultural importance to the local Indigenous people and is recognized as an Aboriginal sacred site. This has placed limitations on past and present mining activities. Today, while some small-scale fossicking may occur, any new or large-scale mining is subject to strict legal protections and requires negotiation with the traditional owners. This intersection of geological rarity and cultural significance defines the enduring story of Ammaroo turquoise