Welo opal

In 2008 a new opal deposit opened up near the village of Wegel Tena in Wollo Province, Ethiopia, and inside a few years it had rearranged the world opal market. The trade calls the material welo opal, and you will also see wollo and wello, all the same place.

The geology is volcanic rather than sedimentary, which is the first thing that separates it from Australian opal. The productive horizon is an opalised rhyolitic ignimbrite, a welded volcanic ash sheet, up to a metre thick, lying on a clay base within a layered volcanic pile. Silica moved through that ash and set as opal in place, which is why the material comes out in flat sheets and nodules following one stratigraphic level rather than in the seams and fossil casts an Australian miner works.

Most of it is white body colour, with brown, fire orange and colourless crystal opal in the mix. The play of colour is often extremely vivid, with broad red and orange patches, and sometimes arranged in parallel columns that the trade calls fingers.

The property everybody argues about is hydrophane. Much of this opal is opaque or translucent dry and turns transparent when it soaks up water, then reverts as it dries. It is a genuinely striking thing to watch and a genuine hazard in a ring, because water, oil and even sweat change the stone's appearance temporarily and can craze it if the cycle is repeated hard. Buy it knowing that. Keep it away from ultrasonic cleaners. Dyeing and impregnation can reverse some of those effects, but the gemmologists who went there say the processes are not yet fully understood, which is why the stability and treatment of Ethiopian opal are still argued over. Ask what has been done to a stone before you pay for it.

The mining is hard hand work. The opal is confined to one thin horizontal layer about 350 metres below the top of the plateau, and crews of ten to twenty miners drive narrow tunnels along it with simple hand tools. A tunnel can run more than 70 metres and narrow to 40 centimetres, and deep inside the air is humid and short of oxygen. A mining association representative told a 2019 gemological expedition that more than 20 tunnels were active in the valley, each producing 50 to 100 kilograms of opal a week, and most of that material goes to Jaipur to be processed.

Wegel Tena is a working commercial field with an established local mining community and it is not somewhere a visitor collects. The reason it is on this map is that it is one of the very few genuinely new gem deposits of the modern era, and inside about a decade one valley there was turning out a tonne or more of opal a week.