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Welo opal and Australian opal: what's different

Guide · · Maedot

Welo opal and Australian opal formed in different rocks and behave differently. Welo (or Wollo) opal, from northern Ethiopia, is mined mostly from one layer in a volcanic sequence, and most of it absorbs water. Australian opal formed in sedimentary rock of the Great Artesian Basin. The trade prices them on different scales: in 2015, fine Ethiopian stones sold wholesale for about $200 to $300 a carat, while the best Australian black opal could fetch tens of thousands of dollars a carat.

Where each forms

At Wegel Tena, the opal sits in one seam less than a metre thick, inside rhyolitic ignimbrite, a rock laid down by volcanic ash flows. The sequence around it is more than 3,000 metres thick and formed about 30 million years ago, as the East African Rift opened. Some Welo opal looks like Australian sedimentary opal, but its structure is opal-CT, the type found in most volcanic opal. GIA's report on its 2018 expedition adds that newer studies point to a strong sedimentary, possibly soil-forming, part in how the opal was deposited.

Almost all Australian opal deposits lie in or along the Great Artesian Basin, in Cretaceous sandstone and claystone. Black opal comes mainly from Lightning Ridge in New South Wales; boulder opal is found in ironstone concretions in Queensland.

Body tone

Welo opal is mostly white, with some crystal, fire and chocolate-brown material. Black opal was found in 2013 at the Stayish mine near Gashena, about 30 km from Wegel Tena; its colour runs evenly through the stone. Some Welo stones show large patches of red and orange, which the 2010 study noted are uncommon in Australian opal. In Australia, body tone defines the most valued class: the Australian Opal Association counts a stone as black opal only if it rates N1 to N4 on its body-tone scale. A black Ethiopian stone calls for more questions than a white one, because porous Welo opal can be darkened by smoke or dye.

Water

Most Welo opal is hydrophane. In the 2010 study, opaque and translucent pieces turned transparent after minutes to an hour in water, gaining up to 10.2% in weight, and returned to their earlier look within a few hours. Before Welo, the authors noted, such behaviour was known in only a few opals, such as those from Opal Butte, Oregon. A wet Welo stone is a poor guide to the same stone dry, and it is the dry stone you are buying.

Durability

All opal is soft and brittle by gem standards: GIA puts it at 5 to 6.5 on the Mohs scale, with toughness from very poor to fair. Within Welo, the 2010 study found white, opaque to translucent opal remarkably durable, while fire and crystal opal broke more easily. In an informal test, five Wegel Tena cabochons dropped 1.5 metres onto concrete were undamaged, and three Australian white opals broke. The samples were too few to rank the two sources.

How the trade prices each

For both, play of colour sets most of the price. GIA ranks brightness first, then the range of colours, and Australian miners and cutters told GIA's field team in 2015 that any red raises value sharply. Australian opal adds body tone, with black opal at the top. Dealers quoted by JCK in 2015 put black opal at a few hundred to tens of thousands of dollars a carat, and Queensland boulder opal at $80 to $13,000.

Ethiopian opal sells far lower. In the same report, small stones cost $3 to $5 a carat wholesale and fine material $200 to $300, after prices had roughly tripled in a few years. GIA's laboratory wrote in 2013 that Wollo opal is often much cheaper than similar opal from Australia and Brazil. One dealer in Australian opal told JCK that Ethiopian supply had all but ended the market for low-end Australian white opal.

Buying opal from us

Our selections show unedited photographs and video marked dry or wet, with weights and pricing, and each parcel lists its stated origin and any treatment or testing on record.

Sources

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