Mezezo opal from Shewa: where it comes from and what it is like

Mezezo opal comes from Yita Ridge, in the Menz Gishe district of North Shewa, about 15 to 20 km north of the town of Mezezo and some 240 km north-east of Addis Ababa. It reached the trade in 1993, fifteen years before the deposits at Wegel Tena in Wollo were found. It forms as nodules in volcanic rock, and its best-known stones have a brown body that the trade calls chocolate opal.

Where it is found

The opal-bearing rock crops out along the north flank of Yita Ridge, at about 2,450 metres. When GIA described the field in 1996, it lay about 8 km off the road between Mezezo and Hirute, a branch of the main road from Addis Ababa to Dessie. The field was estimated to cover at least 7 by 7 km.

The geology

The nodules sit in a continuous layer of welded rhyolitic tuff, about 3 metres thick and close to obsidian in character, between layers of weathered rhyolite. GIA placed the sequence in the Amba Alaji rhyolites, of Miocene age, 8 to 27 million years old. The nodules average about 10 cm across. Like Welo opal, this is volcanic opal, not the sedimentary opal of Australia.

Colour and character

GIA found body colours from near-colourless to white, yellow, orange, grey and brown. Some stones show play of colour face up; many show it only when light passes through them, the effect called contra luz. GIA's 2014 report on a new Wollo deposit describes Mezezo specimens as nodules of reddish-brown volcanic rock with orange, reddish-brown or "chocolate" brown precious opal inside. Gem material is scarce: in nodules gathered from 12 sites, about 15% of the opal was gem quality, and about 1% of that gem opal showed distinct play of colour. Small red-brown grains are common inclusions, often with tiny black grains that may be pyrite.

Stability

In GIA's tests much of the material absorbed water, and some cracked under light and heat: a 7.74 ct piece of yellow rough cracked within 24 hours, 8 cm from a 100-watt lamp. None of the test stones changed further over 96 hours, which suggested to GIA that a few days under a bright lamp can separate durable stones from those that craze easily. Water uptake did not predict cracking. Contra luz and semi-transparent grey pieces gained only 0.5 to 0.6% of their weight when wet, yet cracked badly after two rounds of wetting and drying; semi-translucent yellow and grey pieces gained 11 to 18% and cracked less. Rémi Bornet notes that Welo opal, also hydrophane, tolerates changes in humidity better.

How it reached the market

Stones from the area appeared in the Nairobi gem market in mid-1993. GIA's Gem News section reported them in 1994 and 1995, and the full study followed in 1996. By then only a few hundred kilograms had been produced, all from outcrops at or near the surface, and commercial mining was waiting on surveys of the deposit. Bornet adds that much of it has been exported as rough. Most Ethiopian opal now comes from Wollo, where the Wegel Tena deposit was found in 2008.

Mezezo opal from us

We offer Mezezo opal as well as Welo opal. Each selection states the origin of the opal.

Sources

• Johnson, Kammerling, DeGhionno and Koivula, “Opal from Shewa Province, Ethiopia”, Gems & Gemology, Summer 1996
• Kiefert et al., “New Deposit of Black Opal from Ethiopia”, Gems & Gemology, Winter 2014
• Rondeau et al., “Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia”, Gems & Gemology, Summer 2010
• Bornet, “Opals from Shewa and Welo, Ethiopia”, Le Comptoir Géologique, 2025
• Mindat, “Mezezo Opal Occurrence, North Shewa Zone, Amhara Region, Ethiopia”, undated