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How rough Welo opal is judged before cutting

Guide · · Maedot

Rough Welo opal is judged first on the play of colour it will show once cut: how bright it is, which colours it shows, its pattern, and how much of the stone shows it from which directions. Body tone, what lies behind the visible face, matrix, yield and stability come after. Opal Auctions, a dealer, writes that the trade has no accepted way to grade Ethiopian opal, so the A, AA and AAA on listings are each seller's own grades.

Play of colour

For GIA, play of colour must be vivid to command a high rating, and its range of colours is secondary to its strength. Red is traditionally the most valued colour, then orange, then green. In the first study of Wegel Tena opal, in Gems & Gemology in 2010, the stones generally showed every spectral colour, often with large patches of red and orange.

GIA also looks at how much of the stone shows colour, and counts dead spots, areas with none, against value. Connoisseurs, it notes, prefer large, closely set patches to tiny scattered points. Gem-A adds that value falls if the best colour shows only from the side. Ethiopian opal often carries its colour in rounded columns set in common opal, called digit patterns; in a 2013 study of thousands of stones from the Addis Ababa market, more than 10% showed them.

Body tone and transparency

In the 2010 study, most Wegel Tena opal had a white body and was translucent. Some was pale yellow, a little was orange fire opal or colourless crystal opal, and chocolate brown was rare; GIA's 2019 field report adds reddish brown and black. Some of the finest stones were translucent with a bluish body colour from scattered light.

GIA notes that experts prize transparency in crystal opal, and that a cloudy or milky background lowers value and can signal poor stability. For Gem-A, some transparency raises the value of light opal, because the colour shows deeper in the stone. Body tone counts for less in Welo, Opal Auctions writes, than in Australian opal, where it defines black opal. A dark Welo stone raises the question of smoke or dye.

The face and what lies behind it

Rough is read through its faces: a natural break, or a window, a small polished spot that shows the inside. A window shows the colour at that spot only. In the 2010 study, play of colour went from intense to none within a millimetre in the same piece. Digit columns look like rounded patches seen end-on and like columns from the side, and in one Wollo stone in the 2013 study the colour shifts in layers from red at the base to green at the top.

Each face therefore raises two questions: how deep the colour runs behind it, and from which direction it shows best. In Gems & Gemology in 2016, the cutter Theodore Grussing wrote that he had worked many pieces of rough with brilliant colour from the side and faint colour from the top; the best answer then, he wrote, is small stones cut with the side as the face.

Matrix and yield

Welo opal often cements grains of volcanic debris, which leaves the rough irregular. Host rock attached to the opal is matrix, which GIA says usually lowers the value of a polished opal. Some Wegel Tena rough has a black skin less than 0.1 mm thick, and rarely black minerals fill cracks or form dendrites inside.

All of this is ground away, so it sets the yield. Published yield figures for Welo are scarce. In a 2023 cutting video from Moriarty's Gem Art, a dealer, a 32 ct Welo rough gave a stone of just over 12 ct, a 37% yield the cutter called a little below normal because matrix ran into deep pockets. The planned cut changes the yield as well: GIA notes that fine opal is often cut into irregular shapes that keep the most colour, and commercial white opal into calibrated ovals. Compare yields on dry weight (see carats, grams and kilograms).

Stability

Most Welo opal is hydrophane: it takes in water, turns clearer and returns to its dry look within hours, so judge it dry (see dry or wet). The International Gem Society's cutting notes add that water hides flaws and cracks. In the 2010 study, crazing was confined to transparent material, mostly crystal and fire opal; how to test rough before cutting is in Will my opal craze?

Buying from us

Each parcel in a Maedot selection comes with unedited photographs and video, marked dry or wet, and its weight and price; How buying works explains how to inspect the stones in person, by live video or through samples.

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