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Smoked and dyed Ethiopian opal: how to spot treated dark opal

Guide · · Maedot

Dark Ethiopian opal is often treated. Welo opal is porous enough to take in smoke, dye, or sugar followed by acid, and each can turn white or crystal opal dark or give it a colour it never had. Treated stones have been sold as natural black opal and as new finds from Mexico. The usual signs are colour gathered in pits, scratches and cracks, and colour that sits in a skin at the surface. Natural dark opal also comes from Ethiopia, so colour alone proves nothing; a laboratory can settle it.

Why it takes treatment so easily

Most Welo opal is hydrophane: it absorbs water and other liquids. GIA researchers soaked Wollo opal in permanent-marker ink dissolved in acetone and produced vivid body colours. They expected treated stones to follow, and in October 2011 they were shown bright purple opal said to come from Mexico.

Smoke

In 2011 Stone Group Laboratories in Missouri tested Wollo opal submitted by a television retailer as a new find of natural black opal. The lab reproduced the colour by smoking its own crystal opal. Under magnification, treated stones showed pock marks and patches of concentrated smoke, and their Raman spectra had a carbon peak the untreated stones lacked. The colour survived repolishing and cleaning solvents; fewer than one stone in ten cracked or crazed from the heat.

Sugar and acid

GIA lists sugar treatment among the surface treatments used on opal. In 2014 a Gübelin-led team, describing black opal from a new deposit at the Stayish mine near Gashena in Wollo, noted that sugar-acid darkening sits in a surface layer, while the Stayish opal is dark all the way through with no staining in pits or fissures. Raman carbon peaks alone could not separate it from treated Wegel Tena opal; the authors relied on careful microscopy.

Dye

The purple opal shown to GIA in 2011 was dyed Wollo opal. Colour gathered in pits, scratches and surface-reaching fractures, and one stone immersed in water showed a coloured layer following its surface. Between 2015 and 2017 a designer bought more than 500 carats of blue and pink opal sold as natural Mexican material; GIA found it was dyed, probably Ethiopian, and a soak in 3% hydrogen peroxide bleached the dye. In 2017 GIA also reported a brown dye that looked natural. The colour followed the surface, and the stone held no manganese, which causes natural brown in Ethiopian opal.

Checks before you buy

  • Look at the surface. Under a loupe or microscope, look for dark or coloured concentrations in pits, scratches and cracks, on the back as well as the face.
  • Light it from below. In a 2018 report for the Australian Opal Association comparing 15 stones, every smoked or dyed Ethiopian stone turned transparent and cherry red over a torch held close beneath it. The untreated black opals stayed partly or wholly opaque.
  • Question unusual colours. In 2015 GIA's Shane McClure advised caution with Ethiopian opal that is not white, light yellow, amber or brown, and asking for treatment guarantees in writing. Brown can be dyed as well.
  • Avoid soaking tests. Immersion shows dye layers clearly, but GIA warns that hydrophane opal can crack in liquid. A laboratory can combine microscopy with Raman, infrared and chemical analysis.

Disclosure

The US Federal Trade Commission's jewellery guides treat it as deceptive not to disclose a gemstone treatment that is not permanent, that creates special care needs, or that significantly affects value. The guides apply at every level of the trade, online sales included. The price gap is large: the 2018 report put treated Ethiopian black opal at about $10 a carat wholesale, against up to $5,000 for an untreated black opal that looks the same.

Buying from us

Each parcel in a selection lists its stated origin and any treatment or testing on record, and its photographs and video are unedited and marked dry or wet. You can ask for samples and have them tested.

Sources

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